PRELIMINARY REMARKS—FIRST PART: THE FACTS TO BE CONNECTED UNDER A
SINGLE POINT OF VIEW, NAMELY, THE VARIOUS KINDS OF
REPRODUCTION—RE-GROWTH OF AMPUTATED PARTS—GRAFT-HYBRIDS—THE DIRECT
ACTION OF THE MALE ELEMENT ON THE FEMALE—DEVELOPMENT—THE FUNCTIONAL
INDEPENDENCE OF THE UNITS OF THE
BODY—VARIABILITY—INHERITANCE—REVERSION—SECOND PART: STATEMENT OF THE
HYPOTHESIS—HOW FAR THE NECESSARY ASSUMPTIONS ARE IMPROBABLE—EXPLANATION
BY AID OF THE HYPOTHESIS OF THE SEVERAL CLASSES OF FACTS SPECIFIED IN
THE FIRST PART—CONCLUSION.
In the previous chapters large classes of facts, such as those bearing
on bud-variation, the various forms of inheritance, the causes and laws
of variation, have been discussed; and it is obvious that these
subjects, as well as the several modes of reproduction, stand in some
sort of relation to one another. I have been led, or rather forced, to
form a view which to a certain extent connects these facts by a
tangible method. Every one would wish to explain to himself, even in an
imperfect manner, how it is possible for a character possessed by some
remote ancestor suddenly to reappear in the offspring; how the effects
of increased or decreased use of a limb can be transmitted to the
child; how the male sexual element can act not solely on the ovules,
but occasionally on the mother-form; how a hybrid can be produced by
the union of the cellular tissue of two plants independently of the
organs of generation; how a limb can be reproduced on the exact line of
amputation, with neither too much nor too little added; how the same
organism may be produced by such widely different processes, as budding
and true seminal generation; and, lastly, how of two allied forms, one
passes in the course of its development through the most complex
metamorphoses, and the other does not do so, though when mature both
are alike in every detail of structure. I am aware that my view is
merely a provisional hypothesis or speculation; but until a better one
be advanced, it will serve to bring together a multitude of facts which
are at present left disconnected by any efficient cause. As Whewell,
the historian of the inductive sciences, remarks:—“Hypotheses may often
be of service to science, when they involve a certain portion of
incompleteness, and even of error.” Under this point of view I venture
to advance the hypothesis of Pangenesis, which implies that every
separate part of the whole organisation reproduces itself. So that
ovules, spermatozoa, and pollen-grains,—the fertilised egg or seed, as
well as buds,—include and consist of a multitude of germs thrown off
from each separate part or unit.[1]
In the First Part I will enumerate as briefly as I can the groups of
facts which seem to demand connection; but certain subjects, not
hitherto discussed, must be treated at disproportionate length. In the
Second Part the hypothesis will be given; and after considering how far
the necessary assumptions are in themselves improbable, we shall see
whether it serves to bring under a single point of view the various
facts.
PART I.
Reproduction may be divided into two main classes, namely, sexual and
asexual. The latter is effected in many ways—by the formation of buds
of various kinds, and by fissiparous generation, that is by spontaneous
or artificial division. It is notorious that some of the lower animals,
when cut into many pieces, reproduce so many perfect individuals:
Lyonnet cut a Nais or freshwater worm into nearly forty pieces, and
these all reproduced perfect animals.[2] It is probable that
segmentation could be carried much further in some of the protozoa; and
with some of the lowest plants each cell will reproduce the
parent-form. Johannes Müller thought that there was an important
distinction between gemmation and fission; for in the latter case the
divided portion, however small, is more fully developed than a bud,
which also is a younger formation; but most physiologists are now
convinced that the two processes are essentially alike.[3] Prof. Huxley
remarks, “fission is little more than a peculiar mode of budding,” and
Prof. H. J. Clark shows in detail that there is sometimes “a compromise
between self-division and budding.” When a limb is amputated, or when
the whole body is bisected, the cut extremities are said to bud
forth;[4] and as the papilla, which is first formed, consists of
undeveloped cellular tissue like that forming an ordinary bud, the
expression is apparently correct. We see the connection of the two
processes in another way; for Trembley observed with the hydra, that
the reproduction of the head after amputation was checked as soon as
the animal put forth reproductive gemmæ.[5]
Between the production, by fissiparous generation, of two or more
complete individuals, and the repair of even a very slight injury,
there is so perfect a gradation, that it is impossible to doubt that
the two processes are connected. As at each stage of growth an
amputated part is replaced by one in the same state of development, we
must also follow Sir J. Paget in admitting, “that the powers of
development from the embryo, are identical with those exercised for the
restoration from injuries: in other words, that the powers are the same
by which perfection is first achieved, and by which, when lost, it is
recovered.”[6] Finally, we may conclude that the several forms of
budding, fissiparous generation, the repair of injuries, and
development, are all essentially the results of one and the same power.
_Sexual Generation._—The union of the two sexual elements seems at
first sight to make a broad distinction between sexual and asexual
generation. But the conjugation of algæ, by which process the contents
of two cells unite into a single mass capable of development,
apparently gives us the first step towards sexual union: and
Pringsheim, in his memoir on the pairing of Zoospores,[7] shows that
conjugation graduates into true sexual reproduction. Moreover, the now
well-ascertained cases of Parthenogenesis prove that the distinction
between sexual and asexual generation is not nearly so great as was
formerly thought; for ova occasionally, and even in some cases
frequently, become developed into perfect beings, without the concourse
of the male. With most of the lower animals and even with mammals, the
ova show a trace of parthenogenetic power, for without being fertilised
they pass through the first stages of segmentation.[8] Nor can pseudova
which do not need fertilisation, be distinguished from true ova, as was
first shown by Sir J. Lubbock, and is now admitted by Siebold. So,
again, the germ-balls in the larvæ of Cecidomyia are said by
Leuckart[9] to be formed within the ovarium, but they do not require to
be fertilised. It should also be observed that in sexual generation,
the ovules and the male element have equal power of transmitting every
single character possessed by either parent to their offspring. We see
this clearly when hybrids are paired _inter se,_ for the characters of
both grandparents often appear in the progeny, either perfectly or by
segments. It is an error to suppose that the male transmits certain
characters and the female other characters; although no doubt, from
unknown causes, one sex sometimes has a much stronger power of
transmission than the other.
It has, however, been maintained by some authors that a bud differs
essentially from a fertilised germ, in always reproducing the perfect
character of the parent-stock; whilst fertilised germs give birth to
variable beings. But there is no such broad distinction as this. In the
eleventh chapter numerous cases were advanced showing that buds
occasionally grow into plants having quite new characters; and the
varieties thus produced can be propagated for a length of time by buds,
and occasionally by seed. Nevertheless, it must be admitted that beings
produced sexually are much more liable to vary than those produced
asexually; and of this fact a partial explanation will hereafter be
attempted. The variability in both cases is determined by the same
general causes, and is governed by the same laws. Hence new varieties
arising from buds cannot be distinguished from those arising from seed.
Although bud-varieties usually retain their character during successive
bud-generations, yet they occasionally revert, even after a long series
of bud-generations, to their former character. This tendency to
reversion in buds, is one of the most remarkable of the several points
of agreement between the offspring from bud and seminal reproduction.
But there is one difference between organisms produced sexually and
asexually, which is very general. The former pass in the course of
their development from a very low stage to their highest stage, as we
see in the metamorphoses of insects and of many other animals, and in
the concealed metamorphoses of the vertebrata. Animals propagated
asexually by buds or fission, on the other hand, commence their
development at that stage at which the budding or self-dividing animal
may happen to be, and therefore do not pass through some of the lower
developmental stages.[10] Afterwards, they often advance in
organisation, as we see in the many cases of “alternate generation.” In
thus speaking of alternate generation, I follow those naturalists who
look at this process as essentially one of internal budding or of
fissiparous generation. Some of the lower plants, however, such as
mosses and certain algæ, according to Dr. L. Radlkofer,[11] when
propagated asexually, do undergo a retrogressive metamorphosis. As far
as the final cause is concerned, we can to a certain extent understand
why beings propagated by buds should not pass through all the early
stages of development; for with each organism the structure acquired at
each stage must be adapted to its peculiar habits; and if there are
places for the support of many individuals at some one stage, the
simplest plan will be that they should be multiplied at this stage, and
not that they should first retrograde in their development to an
earlier or simpler structure, which might not be fitted for the then
surrounding conditions.
From the several foregoing considerations we may conclude that the
difference between sexual and asexual generation is not nearly so great
as at first appears; the chief difference being that an ovule cannot
continue to live and to be fully developed unless it unites with the
male element; but even this difference is far from invariable, as shown
by the many cases of parthenogenesis. We are therefore naturally led to
inquire what the final cause can be of the necessity in ordinary
generation for the concourse of the two sexual elements.
Seeds and ova are often highly serviceable as the means of
disseminating plants and animals, and of preserving them during one or
more seasons in a dormant state; but unimpregnated seeds or ova, and
detached buds, would be equally serviceable for both purposes. We can,
however, indicate two important advantages gained by the concourse of
the two sexes, or rather of two individuals belonging to opposite
sexes; for, as I have shown in a former chapter, the structure of every
organism appears to be especially adapted for the concurrence, at least
occasionally, of two individuals. When species are rendered highly
variable by changed conditions of life, the free intercrossing of the
varying individuals tends to keep each form fitted for its proper place
in nature; and crossing can be effected only by sexual generation; but
whether the end thus gained is of sufficient importance to account for
the first origin of sexual intercourse is extremely doubtful. Secondly,
I have shown from a large body of facts, that, as a slight change in
the conditions of life is beneficial to each creature, so, in an
analogous manner, is the change effected in the germ by sexual union
with a distinct individual; and I have been led, from observing the
many widely-extended provisions throughout nature for this purpose, and
from the greater vigour of crossed organisms of all kinds, as proved by
direct experiments, as well as from the evil effects of close
interbreeding when long continued, to believe that the advantage thus
gained is very great.
Why the germ, which before impregnation undergoes a certain amount of
development, ceases to progress and perishes, unless it be acted on by
the male element; and why conversely the male element, which in the
case of some insects is enabled to keep alive for four or five years,
and in the case of some plants for several years, likewise perishes,
unless it acts on or unites with the germ, are questions which cannot
be answered with certainty. It is, however, probable that both sexual
elements perish, unless brought into union, simply from including too
little formative matter for independent development. Quatrefages has
shown in the case of the Teredo,[12] as did formerly Prevost and Dumas
with other animals, that more than one spermatozoon is requisite to
fertilise an ovum. This has likewise been shown by Newport,[13] who
proved by numerous experiments, that, when a very small number of
spermatozoa are applied to the ova of Batrachians, they are only
partially impregnated, and an embryo is never fully developed. The rate
also of the segmentation of the ovum is determined by the number of the
spermatozoa. With respect to plants, nearly the same results were
obtained by Kölreuter and Gärtner. This last careful observer, after
making successive trials on a Malva with more and more pollen-grains,
found,[14] that even thirty grains did not fertilise a single seed; but
when forty grains were applied to the stigma, a few seeds of small size
were formed. In the case of Mirabilis the pollen grains are
extraordinarily large, and the ovarium contains only a single ovule;
and these circumstances led Naudin[15] to make the following
experiments: a flower was fertilised by three grains and succeeded
perfectly; twelve flowers were fertilised by two grains, and seventeen
flowers by a single grain, and of these one flower alone in each lot
perfected its seed: and it deserves especial notice that the plants
produced by these two seeds never attained their proper dimensions, and
bore flowers of remarkably small size. From these facts we clearly see
that the quantity of the peculiar formative matter which is contained
within the spermatozoa and pollen-grains is an all-important element in
the act of fertilisation, not only for the full development of the
seed, but for the vigour of the plant produced from such seed. We see
something of the same kind in certain cases of parthenogenesis, that
is, when the male element is wholly excluded; for M. Jourdan[16] found
that, out of about 58,000 eggs laid by unimpregnated silk-moths, many
passed through their early embryonic stages, showing that they were
capable of self-development, but only twenty-nine out of the whole
number produced caterpillars. The same principle of quantity seems to
hold good even in artificial fissiparous reproduction, for Hackel[17]
found that by cutting the segmented and fertilised ova or larva of
Siphonophoræ (jelly-fishes) into pieces, the smaller the pieces were,
the slower was the rate of development, and the larvæ thus produced
were by so much the more imperfect and inclined to monstrosity. It
seems, therefore, probable that with the separate sexual elements
deficient quantity of formative matter is the main cause of their not
having the capacity for prolonged existence and development, unless
they combine and thus increase each other’s bulk. The belief that it is
the function of the spermatozoa to communicate life to the ovule seems
a strange one, seeing that the unimpregnated ovule is already alive and
generally undergoes a certain amount of independent development. Sexual
and asexual reproduction are thus seen not to differ essentially; and
we have already shown that asexual reproduction, the power of re-growth
and development are all parts of one and the same great law.
_Re-growth of amputated parts._—This subject deserves a little further
discussion. A multitude of the lower animals and some vertebrates
possess this wonderful power. For instance, Spallanzani cut off the
legs and tail of the same salamander six times successively, and
Bonnet[18] did so eight times; and on each occasion the limbs were
reproduced on the exact line of amputation, with no part deficient or
in excess. An allied animal, the axolotl, had a limb bitten off, which
was reproduced in an abnormal condition, but when this was amputated it
was replaced by a perfect limb.[19] The new limbs in these cases bud
forth, and are developed in the same manner as during the regular
development of a young animal. For instance, with the _Amblystoma
lurida,_ three toes are first developed, then the fourth, and on the
hind-feet the fifth, and so it is with a reproduced limb.[20]
The power of re-growth is generally much greater during the youth of an
animal or during the earlier stages of its development than during
maturity. The larvæ or tadpoles of the Batrachians are capable of
reproducing lost members, but not so the adults.[21] Mature insects
have no power of re-growth, excepting in one order, whilst the larvæ of
many kinds have this power. Animals low in the scale are able, as a
general rule, to reproduce lost parts far more easily than those which
are more highly organised. The myriapods offer a good illustration of
this rule; but there are some strange exceptions to it—thus Nemerteans,
though lowly organised, are said to exhibit little power of re-growth.
With the higher vertebrata, such as birds and mammals, the power is
extremely limited.[22]
In the case of those animals which may be bisected or chopped into
pieces, and of which every fragment will reproduce the whole, the power
of re-growth must be diffused throughout the whole body. Nevertheless
there seems to be much truth in the view maintained by Prof.
Lessona,[23] that this capacity is generally a localised and special
one, serving to replace parts which are eminently liable to be lost in
each particular animal. The most striking case in favour of this view,
is that the terrestrial salamander, according to Lessona, cannot
reproduce lost parts, whilst another species of the same genus, the
aquatic salamander, has extraordinary powers of re-growth, as we have
just seen; and this animal is eminently liable to have its limbs, tail,
eyes and jaws bitten off by other tritons.[24] Even with the aquatic
salamander the capacity is to a certain extent localised, for when M.
Philipeaux[25] extirpated the entire fore limb together with the
scapula, the power of re-growth was completely lost. It is also a
remarkable fact, standing in opposition to a very general rule, that
the young of the aquatic salamander do not possess the power of
repairing their limbs in an equal degree with the adults[26] but I do
not know that they are more active, or can otherwise better escape the
loss of their limbs, than the adults. The walking-stick insect,
_Diapheromera femorata,_ like other insects of the same order, can
reproduce its legs in the mature state, and these from their great
length must be liable to be lost: but the capacity is localised (as in
the case of the salamander), for Dr. Scudder found,[27] that if the
limb was removed within the trochanto-femoral articulation, it was
never renewed. When a crab is seized by one of its legs, this is thrown
off at the basal joint, being afterwards replaced by a new leg; and it
is generally admitted that this is a special provision for the safety
of the animal. Lastly, with gasteropod molluscs, which are well known
to have the power of reproducing their heads, Lessona shows that they
are very liable to have their heads bitten off by fishes; the rest of
the body being protected by the shell. Even with plants we see
something of the same kind, for non-deciduous leaves and young stems
have no power of re-growth, these parts being easily replaced by growth
from new buds; whilst the bark and subjacent tissues of the trunks of
trees have great power of re-growth, probably on account of their
increase in diameter, and of their liability to injury from being
gnawed by animals.
_Graft-hybrids._—It is well known from innumerable trials made in all
parts of the world, that buds may be inserted into a stock, and that
the plants thus raised are not affected in a greater degree than can be
accounted for by changed nutrition. Nor do the seedlings raised from
such inserted buds partake of the character of the stock, though they
are more liable to vary than are seedlings from the same variety
growing on its own roots. A bud, also, may sport into a new and
strongly-marked variety without any other bud on the same plant being
in the least degree affected. We may therefore infer, in accordance
with the common view, that each bud is a distinct individual, and that
its formative elements do not spread beyond the parts subsequently
developed from it. Nevertheless, we have seen in the abstract on
graft-hybridisation in the eleventh chapter that buds certainly include
formative matter, which can occasionally combine with that included in
the tissues of a distinct variety or species; a plant intermediate
between the two parent-forms being thus produced. In the case of the
potato we have seen that the tubers produced from a bud of one kind
inserted into another are intermediate in colour, size, shape and state
of surface; that the stems, foliage, and even certain constitutional
peculiarities, such as precocity, are likewise intermediate. With these
well-established cases, the evidence that graft-hybrids have also been
produced with the laburnum, orange, vine, rose, etc., seems sufficient.
But we do not know under what conditions this rare form of reproduction
is possible. From these several cases we learn the important fact that
formative elements capable of blending with those of a distinct
individual (and this is the chief characteristic of sexual generation),
are not confined to the reproductive organs, but are present in the
buds and cellular tissue of plants; and this is a fact of the highest
physiological importance.
_Direct Action of the Male Element on the Female._—In the eleventh
chapter, abundant proofs were given that foreign pollen occasionally
affects in a direct manner the mother-plant. Thus, when Gallesio
fertilised an orange-flower with pollen from the lemon, the fruit bore
stripes of perfectly characterised lemon-peel. With peas, several
observers have seen the colour of the seed-coats and even of the pod
directly affected by the pollen of a distinct variety. So it has been
with the fruit of the apple, which consists of the modified calyx and
upper part of the flower-stalk. In ordinary cases these parts are
wholly formed by the mother-plant. We here see that the formative
elements included within the male element or pollen of one variety can
affect and hybridise, not the part which they are properly adapted to
affect, namely, the ovules, but the partially-developed tissues of a
distinct variety or species. We are thus brought half-way towards a
graft-hybrid, in which the formative elements included within the
tissues of one individual combine with those included in the tissues of
a distinct variety or species, thus giving rise to a new and
intermediate form, independently of the male or female sexual organs.
With animals which do not breed until nearly mature, and of which all
the parts are then fully developed, it is hardly possible that the male
element should directly affect the female. But we have the analogous
and perfectly well-ascertained case of the male element affecting (as
with the quagga and Lord Morton’s mare) the female or her ova, in such
a manner that when she is impregnated by another male her offspring are
affected and hybridised by the first male. The explanation would be
simple if the spermatozoa could keep alive within the body of the
female during the long interval which has sometimes elapsed between the
two acts of impregnation; but no one will suppose that this is possible
with the higher animals.
_Development._—The fertilised germ reaches maturity by a vast number of
changes: these are either slight and slowly effected, as when the child
grows into the man, or are great and sudden, as with the metamorphoses
of most insects. Between these extremes we have every gradation, even
within the same class; thus, as Sir J. Lubbock has shown[28] there is
an Ephemerous insect which moults above twenty times, undergoing each
time a slight but decided change of structure; and these changes, as he
further remarks, probably reveal to us the normal stages of
development, which are concealed and hurried through or suppressed in
most other insects. In ordinary metamorphoses, the parts and organs
appear to become changed into the corresponding parts in the next stage
of development; but there is another form of development, which has
been called by Professor Owen metagenesis. In this case “the new parts
are not moulded upon the inner surface of the old ones. The plastic
force has changed its course of operation. The outer case, and all that
gave form and character to the precedent individual, perish and are
cast off; they are not changed into the corresponding parts of the new
individual. These are due to a new and distinct developmental process,”
etc.[29] Metamorphosis, however, graduates so insensibly, into
metagenesis, that the two processes cannot be distinctly separated. For
instance, in the last change which Cirripedes undergo, the alimentary
canal and some other organs are moulded on pre-existing parts; but the
eyes of the old and the young animal are developed in entirely
different parts of the body; the tips of the mature limbs are formed
within the larval limbs, and may be said to be metamorphosed from them;
but their basal portions and the whole thorax are developed in a plane
at right angles to the larval limbs and thorax; and this may be called
metagenesis. The metagenetic process is carried to an extreme point in
the development of some Echinoderms, for the animal in the second stage
of development is formed almost like a bud within the animal of the
first stage, the latter being then cast off like an old vestment, yet
sometimes maintaining for a short period an independent vitality.[30]
If, instead of a single individual, several were to be thus developed
metagenetically within a pre-existing form, the process would be called
one of alternate generation. The young thus developed may either
closely resemble the encasing parent-form, as with the larvæ of
Cecidomyia, or may differ to an astonishing degree, as with many
parasitic worms and jelly-fishes; but this does not make any essential
difference in the process, any more than the greatness or abruptness of
the change in the metamorphoses of insects.
The whole question of development is of great importance for our
present subject. When an organ, the eye, for instance, is
metagenetically formed in a part of the body where during the previous
stage of development no eye existed, we must look at it as a new and
independent growth. The absolute independence of new and old
structures, although corresponding in structure and function, is still
more obvious when several individuals are formed within a previous
form, as in the cases of alternate generation. The same important
principle probably comes largely into play even in the case of
apparently continuous growth, as we shall see when we consider the
inheritance of modifications at corresponding ages.
We are led to the same conclusion, namely, the independence of parts
successively developed, by another and quite distinct group of facts.
It is well known that many animals belonging to the same order, and
therefore not differing widely from each other, pass through an
extremely different course of development. Thus certain beetles, not in
any way remarkably different from others of the same order, undergo
what has been called a hyper-metamorphosis—that is, they pass through
an early stage wholly different from the ordinary grub-like larva. In
the same sub-order of crabs, namely, the Macroura, as Fritz Müller
remarks, the river cray-fish is hatched under the same form which it
ever afterwards retains; the young lobster has divided legs, like a
Mysis; the Palæmon appears under the form of a Zoea, and Peneus under
the Nauplius-form; and how wonderfully these larval forms differ from
one another, is known to every naturalist.[31] Some other crustaceans,
as the same author observes, start from the same point and arrive at
nearly the same end, but in the middle of their development are widely
different from one another. Still more striking cases could be given
with respect to the Echinodermata. With the Medusæ or jelly-fishes
Professor Allman observes, “The classification of the Hydroida would be
a comparatively simple task if, as has been erroneously asserted,
generically-identical medusoids always arose from generically-identical
polypoids; and, on the other hand, that generically-identical polypoids
always gave origin to generically-identical medusoids.” So again, Dr.
Strethill Wright remarks, “In the life-history of the Hydroidæ any
phase, planuloid, polypoid, or medusoid, may be absent.”[32]
According to the belief now generally accepted by our best naturalists,
all the members of the same order or class, for instance, the Medusæ or
the Macrourous crustaceans, are descended from a common progenitor.
During their descent they have diverged much in structure, but have
retained much in common; and this has occurred, though they have passed
through and still pass through marvellously different metamorphoses.
This fact well illustrates how independent each structure is from that
which precedes and that which follows it in the course of development.
_The Functional Independence of the Elements or Units of the
Body._—Physiologists agree that the whole organism consists of a
multitude of elemental parts, which are to a great extent independent
of one another. Each organ, says Claude Bernard,[33] has its proper
life, its autonomy; it can develop and reproduce itself independently
of the adjoining tissues. A great German authority, Virchow,[34]
asserts still more emphatically that each system consists of an
“enormous mass of minute centres of action. . . . Every element has its
own special action, and even though it derive its stimulus to activity
from other parts, yet alone effects the actual performance of duties. .
. . Every single epithelial and muscular fibre-cell leads a sort of
parasitical existence in relation to the rest of the body. . . . Every
single bone-corpuscle really possesses conditions of nutrition peculiar
to itself.” Each element, as Sir J. Paget remarks, lives its appointed
time and then dies, and is replaced after being cast off or
absorbed.[35] I presume that no physiologist doubts that, for instance,
each bone-corpuscle of the finger differs from the corresponding
corpuscle in the corresponding joint of the toe; and there can hardly
be a doubt that even those on the corresponding sides of the body
differ, though almost identical in nature. This near approach to
identity is curiously shown in many diseases in which the same exact
points on the right and left sides of the body are similarly affected;
thus Sir J. Paget[36] gives a drawing of a diseased pelvis, in which
the bone has grown into a most complicated pattern, but “there is not
one spot or line on one side which is not represented, as exactly as it
would be in a mirror, on the other.”
Many facts support this view of the independent life of each minute
element of the body. Virchow insists that a single bone-corpuscle or a
single cell in the skin may become diseased. The spur of a cock, after
being inserted into the ear of an ox, lived for eight years, and
acquired a weight of 396 grammes (nearly fourteen ounces), and the
astonishing length of twenty-four centimetres, or about nine inches; so
that the head of the ox appeared to bear three horns.[37] The tail of a
pig has been grafted into the middle of its back, and reacquired
sensibility. Dr. Ollier[38] inserted a piece of periosteum from the
bone of a young dog under the skin of a rabbit, and true bone was
developed. A multitude of similar facts could be given. The frequent
presence of hairs and of perfectly developed teeth, even teeth of the
second dentition, in ovarian tumours,[39] are facts leading to the same
conclusion. Mr. Lawson Tait refers to a tumour in which “over 300 teeth
were found, resembling in many respects milk-teeth;” and to another
tumour, “full of hair which had grown and been shed from one little
spot of skin not bigger than the tip of my little finger. The amount of
hair in the sac, had it grown from a similarly sized area of the scalp,
would have taken almost a lifetime to grow and be shed.”
Whether each of the innumerable autonomous elements of the body is a
cell or the modified product of a cell, is a more doubtful question,
even if so wide a definition be given to the term, as to include
cell-like bodies without walls and without nuclei.[40] The doctrine of
_omnis cellula e cellulâ_ is admitted for plants, and widely prevails
with respect to animals.[41] Thus Virchow, the great supporter of the
cellular theory, whilst allowing that difficulties exist, maintains
that every atom of tissue is derived from cells, and these from
pre-existing cells, and these primarily from the egg, which he regards
as a great cell. That cells, still retaining the same nature, increase
by self-division or proliferation, is admitted by every one. But when
an organism undergoes great changes of structure during development,
the cells, which at each stage are supposed to be directly derived from
previously existing cells, must likewise be greatly changed in nature;
this change is attributed by the supporters of the cellular doctrine to
some inherent power which the cells possess, and not to any external
agency. Others maintain that cells and tissues of all kinds may be
formed, independently of pre-existing cells, from plastic lymph or
blastema. Whichever view may be correct, every one admits that the body
consists of a multitude of organic units, all of which possess their
own proper attributes, and are to a certain extent independent of all
others. Hence it will be convenient to use indifferently the terms
cells or organic units, or simply units.
_Variability and Inheritance._—We have seen in the twenty-second
chapter that variability is not a principle co-ordinate with life or
reproduction, but results from special causes, generally from changed
conditions acting during successive generations. The fluctuating
variability thus induced is apparently due in part to the sexual system
being easily affected, so that it is often rendered impotent; and when
not so seriously affected, it often fails in its proper function of
transmitting truly the characters of the parents to the offspring. But
variability is not necessarily connected with the sexual system, as we
see in the cases of bud-variation. Although we are seldom able to trace
the nature of the connection, many deviations of structure no doubt
result from changed conditions acting directly on the organisation,
independently of the reproductive system. In some instances we may feel
sure of this, when all, or nearly all the individuals which have been
similarly exposed are similarly and definitely affected, of which
several instances have been given. But it is by no means clear why the
offspring should be affected by the exposure of the parents to new
conditions, and why it is necessary in most cases that several
generations should have been thus exposed.
How, again, can we explain the inherited effects of the use or disuse
of particular organs? The domesticated duck flies less and walks more
than the wild duck, and its limb-bones have become diminished and
increased in a corresponding manner in comparison with those of the
wild duck. A horse is trained to certain paces, and the colt inherits
similar consensual movements. The domesticated rabbit becomes tame from
close confinement; the dog, intelligent from associating with man; the
retriever is taught to fetch and carry; and these mental endowments and
bodily powers are all inherited. Nothing in the whole circuit of
physiology is more wonderful. How can the use or disuse of a particular
limb or of the brain affect a small aggregate of reproductive cells,
seated in a distant part of the body, in such a manner that the being
developed from these cells inherits the characters of either one or
both parents? Even an imperfect answer to this question would be
satisfactory.
In the chapters devoted to inheritance it was shown that a multitude of
newly acquired characters, whether injurious or beneficial, whether of
the lowest or highest vital importance, are often faithfully
transmitted—frequently even when one parent alone possesses some new
peculiarity; and we may on the whole conclude that inheritance is the
rule, and non-inheritance the anomaly. In some instances a character is
not inherited, from the conditions of life being directly opposed to
its development; in many instances, from the conditions incessantly
inducing fresh variability, as with grafted fruit-trees and
highly-cultivated flowers. In the remaining cases the failure may be
attributed to reversion, by which the child resembles its grandparents
or more remote progenitors, instead of its parents.
Inheritance is governed by various laws. Characters which first appear
at any particular age tend to reappear at a corresponding age. They
often become associated with certain seasons of the year, and reappear
in the offspring at a corresponding season. If they appear rather late
in life in one sex, they tend to reappear exclusively in the same sex
at the same period of life.
The principle of reversion, recently alluded to, is one of the most
wonderful of the attributes of Inheritance. It proves to us that the
transmission of a character and its development, which ordinarily go
together and thus escape discrimination, are distinct powers; and these
powers in some cases are even antagonistic, for each acts alternately
in successive generations. Reversion is not a rare event, depending on
some unusual or favourable combination of circumstances, but occurs so
regularly with crossed animals and plants, and so frequently with
uncrossed breeds, that it is evidently an essential part of the
principle of inheritance. We know that changed conditions have the
power of evoking long-lost characters, as in the case of animals
becoming feral. The act of crossing in itself possesses this power in a
high degree. What can be more wonderful than that characters, which
have disappeared during scores, or hundreds, or even thousands of
generations, should suddenly reappear perfectly developed, as in the
case of pigeons and fowls, both when purely bred and especially when
crossed; or as with the zebrine stripes on dun-coloured horses, and
other such cases? Many monstrosities come under this same head, as when
rudimentary organs are redeveloped, or when an organ which we must
believe was possessed by an early progenitor of the species, but of
which not even a rudiment is left, suddenly reappears, as with the
fifth stamen in some Scrophulariaceæ. We have already seen that
reversion acts in bud-reproduction; and we know that it occasionally
acts during the growth of the same individual animal, especially, but
not exclusively, if of crossed parentage,—as in the rare cases
described of fowls, pigeons, cattle, and rabbits, which have reverted
to the colours of one of their parents or ancestors as they advanced in
years.
We are led to believe, as formerly explained, that every character
which occasionally reappears is present in a latent form in each
generation, in nearly the same manner as in male and female animals the
secondary characters of the opposite sex lie latent and ready to be
evolved when the reproductive organs are injured. This comparison of
the secondary sexual characters which lie latent in both sexes, with
other latent characters, is the more appropriate from the case recorded
of a Hen, which assumed some of the masculine characters, not of her
own race, but of an early progenitor; she thus exhibited at the same
time the redevelopment of latent characters of both kinds. In every
living creature we may feel assured that a host of long-lost characters
lie ready to be evolved under proper conditions. How can we make
intelligible and connect with other facts, this wonderful and common
capacity of reversion,—this power of calling back to life long-lost
characters?
PART II.
I have now enumerated the chief facts which every one would desire to
see connected by some intelligible bond. This can be done, if we make
the following assumptions, and much may be advanced in favour of the
chief one. The secondary assumptions can likewise be supported by
various physiological considerations. It is universally admitted that
the cells or units of the body increase by self-division or
proliferation, retaining the same nature, and that they ultimately
become converted into the various tissues and substances of the body.
But besides this means of increase I assume that the units throw off
minute granules which are dispersed throughout the whole system; that
these, when supplied with proper nutriment, multiply by self-division,
and are ultimately developed into units like those from which they were
originally derived. These granules may be called gemmules. They are
collected from all parts of the system to constitute the sexual
elements, and their development in the next generation forms a new
being; but they are likewise capable of transmission in a dormant state
to future generations and may then be developed. Their development
depends on their union with other partially developed or nascent cells
which precede them in the regular course of growth. Why I use the term
union, will be seen when we discuss the direct action of pollen on the
tissues of the mother-plant. Gemmules are supposed to be thrown off by
every unit, not only during the adult state, but during each stage of
development of every organism; but not necessarily during the continued
existence of the same unit. Lastly, I assume that the gemmules in their
dormant state have a mutual affinity for each other, leading to their
aggregation into buds or into the sexual elements. Hence, it is not the
reproductive organs or buds which generate new organisms, but the units
of which each individual is composed. These assumptions constitute the
provisional hypothesis which I have called Pangenesis. Views in many
respects similar have been propounded by various authors.[42]
Before proceeding to show, firstly, how far these assumptions are in
themselves probable, and secondly, how far they connect and explain the
various groups of facts with which we are concerned, it may be useful
to give an illustration, as simple as possible, of the hypothesis. If
one of the Protozoa be formed, as it appears under the microscope, of a
small mass of homogeneous gelatinous matter, a minute particle or
gemmule thrown off from any part and nourished under favourable
circumstances would reproduce the whole; but if the upper and lower
surfaces were to differ in texture from each other and from the central
portion, then all three parts would have to throw off gemmules, which
when aggregated by mutual affinity would form either buds or the sexual
elements, and would ultimately be developed into a similar organism.
Precisely the same view may be extended to one of the higher animals;
although in this case many thousand gemmules must be thrown off from
the various parts of the body at each stage of development; these
gemmules being developed in union with pre-existing nascent cells in
due order of succession.
Physiologists maintain, as we have seen, that each unit of the body,
though to a large extent dependent on others, is likewise to a certain
extent independent or autonomous, and has the power of increasing by
self-division. I go one step further, and assume that each unit casts
off free gemmules which are dispersed throughout the system, and are
capable under proper conditions of being developed into similar units.
Nor can this assumption be considered as gratuitous and improbable. It
is manifest that the sexual elements and buds include formative matter
of some kind, capable of development; and we now know from the
production of graft-hybrids that similar matter is dispersed throughout
the tissues of plants, and can combine with that of another and
distinct plant, giving rise to a new being, intermediate in character.
We know also that the male element can act directly on the partially
developed tissues of the mother-plant, and on the future progeny of
female animals. The formative matter which is thus dispersed throughout
the tissues of plants, and which is capable of being developed into
each unit or part, must be generated there by some means; and my chief
assumption is that this matter consists of minute particles or gemmules
cast off from each unit or cell.[43]
But I have further to assume that the gemmules in their undeveloped
state are capable of largely multiplying themselves by self-division,
like independent organisms. Delpino insists that to “admit of
multiplication by fissiparity in corpuscles, analogous to seeds or buds
. . . is repugnant to all analogy.” But this seems a strange objection,
as Thuret[44] has seen the zoospore of an alga divide itself, and each
half germinated. Haeckel divided the segmented ovum of a siphonophora
into many pieces, and these were developed. Nor does the extreme
minuteness of the gemmules, which can hardly differ much in nature from
the lowest and simplest organisms, render it improbable that they
should grow and multiply. A great authority, Dr. Beale,[45] says “that
minute yeast cells are capable of throwing off buds or gemmules, much
less than the 1/100000 of an inch in diameter;” and these he thinks are
“capable of subdivision practically ad infinitum.”
A particle of small-pox matter, so minute as to be borne by the wind,
must multiply itself many thousandfold in a person thus inoculated; and
so with the contagious matter of scarlet fever.[46] It has recently
been ascertained[47] that a minute portion of the mucous discharge from
an animal affected with rinderpest, if placed in the blood of a healthy
ox, increases so fast that in a short space of time “the whole mass of
blood, weighing many pounds, is infected, and every small particle of
that blood contains enough poison to give, within less than forty-eight
hours, the disease to another animal.”
The retention of free and undeveloped gemmules in the same body from
early youth to old age will appear improbable, but we should remember
how long seeds lie dormant in the earth and buds in the bark of a tree.
Their transmission from generation to generation will appear still more
improbable; but here again we should remember that many rudimentary and
useless organs have been transmitted during an indefinite number of
generations. We shall presently see how well the long-continued
transmission of undeveloped gemmules explains many facts.
As each unit, or group of similar units, throughout the body, casts off
its gemmules, and as all are contained within the smallest ovule, and
within each spermatozoon or pollen-grain, and as some animals and
plants produce an astonishing number of pollen-grains and ovules,[48]
the number and minuteness of the gemmules must be something
inconceivable. But considering how minute the molecules are, and how
many go to the formation of the smallest granule of any ordinary
substance, this difficulty with respect to the gemmules is not
insuperable. From the data arrived at by Sir W. Thomson, my son George
finds that a cube of 1/10000 of an inch of glass or water must consist
of between 16 million millions, and 131 thousand million million
molecules. No doubt the molecules of which an organism is formed are
larger, from being more complex, than those of an inorganic substance,
and probably many molecules go to the formation of a gemmule; but when
we bear in mind that a cube of 1/10000 of an inch is much smaller than
any pollen-grain, ovule or bud, we can see what a vast number of
gemmules one of these bodies might contain.
The gemmules derived from each part or organ must be thoroughly
dispersed throughout the whole system. We know, for instance, that even
a minute fragment of a leaf of a Begonia will reproduce the whole
plant; and that if a fresh-water worm is chopped into small pieces,
each will reproduce the whole animal. Considering also the minuteness
of the gemmules and the permeability of all organic tissues, the
thorough dispersion of the gemmules is not surprising. That matter may
be readily transferred without the aid of vessels from part to part of
the body, we have a good instance in a case recorded by Sir J. Paget of
a lady, whose hair lost its colour at each successive attack of
neuralgia and recovered it again in the course of a few days. With
plants, however, and probably with compound animals, such as corals,
the gemmules do not ordinarily spread from bud to bud, but are confined
to the parts developed from each separate bud; and of this fact no
explanation can be given.
The assumed elective affinity of each gemmule for that particular cell
which precedes it in due order of development is supported by many
analogies. In all ordinary cases of sexual reproduction, the male and
female elements certainly have a mutual affinity for each other: thus,
it is believed that about ten thousand species of Compositæ exist, and
there can be no doubt that if the pollen of all these species could be
simultaneously or successively placed on the stigma of any one species,
this one would elect with unerring certainty its own pollen. This
elective capacity is all the more wonderful, as it must have been
acquired since the many species of this great group of plants branched
off from a common progenitor. On any view of the nature of sexual
reproduction, the formative matter of each part contained within the
ovules and the male element act on each other by some law of special
affinity, so that corresponding parts affect one another; thus, a calf
produced from a short-horned cow by a long-horned bull has its horns
affected by the union of the two forms, and the offspring from two
birds with differently coloured tails have their tails affected.
The various tissues of the body plainly show, as many physiologists
have insisted,[49] an affinity for special organic substances, whether
natural or foreign to the body. We see this in the cells of the kidneys
attracting urea from the blood; in curare affecting certain nerves;
_Lytta vesicatoria_ the kidneys; and the poisonous matter of various
diseases, as small-pox, scarlet-fever, hooping-cough, glanders, and
hydrophobia, affecting certain definite parts of the body.
It has also been assumed that the development of each gemmule depends
on its union with another cell or unit which has just commenced its
development, and which precedes it in due order of growth. That the
formative matter within the pollen of plants, which by our hypothesis
consists of gemmules, can unite with and modify the partially developed
cells of the mother-plant, we have clearly seen in the section devoted
to this subject. As the tissues of plants are formed, as far as is
known, only by the proliferation of pre-existing cells, we must
conclude that the gemmules derived from the foreign pollen do not
become developed into new and separate cells, but penetrate and modify
the nascent cells of the mother-plant. This process may be compared
with what takes place in the act of ordinary fertilisation, during
which the contents of the pollen-tubes penetrate the closed embryonic
sac within the ovule, and determine the development of the embryo.
According to this view, the cells of the mother-plant may almost
literally be said to be fertilised by the gemmules derived from the
foreign pollen. In this case and in all others the proper gemmules must
combine in due order with pre-existing nascent cells, owing to their
elective affinities. A slight difference in nature between the gemmules
and the nascent cells would be far from interfering with their mutual
union and development, for we well know in the case of ordinary
reproduction that such slight differentiation in the sexual elements
favours in a marked manner their union and subsequent development, as
well as the vigour of the offspring thus produced.
Thus far we have been able by the aid of our hypothesis to throw some
obscure light on the problems which have come before us; but it must be
confessed that many points remain altogether doubtful. Thus it is
useless to speculate at what period of development each unit of the
body casts off its gemmules, as the whole subject of the development of
the various tissues is as yet far from clear. We do not know whether
the gemmules are merely collected by some unknown means at certain
seasons within the reproductive organs, or whether after being thus
collected they rapidly multiply there, as the flow of blood to these
organs at each breeding season seems to render probable. Nor do we know
why the gemmules collect to form buds in certain definite places,
leading to the symmetrical growth of trees and corals. We have no means
of deciding whether the ordinary wear and tear of the tissues is made
good by means of gemmules, or merely by the proliferation of
pre-existing cells. If the gemmules are thus consumed, as seems
probable from the intimate connection between the repair of waste,
re-growth, and development, and more especially from the periodical
changes which many male animals undergo in colour and structure, then
some light would be thrown on the phenomena of old age, with its
lessened power of reproduction and of the repair of injuries, and on
the obscure subject of longevity. The fact of castrated animals, which
do not cast off innumerable gemmules in the act of reproduction, not
being longer-lived than perfect males, seems opposed to the belief that
gemmules are consumed in the ordinary repair of wasted tissues; unless
indeed the gemmules after being collected in small numbers within the
reproductive organs are there largely multiplied.[50]
That the same cells or units may live for a long period and continue
multiplying without being modified by their union with free gemmules of
any kind, is probable from such cases as that of the spur of a cock
which grew to an enormous size when grafted into the ear of an ox. How
far units are modified during their normal growth by absorbing peculiar
nutriment from the surrounding tissues, independently of their union
with gemmules of a distinct nature, is another doubtful point.[51] We
shall appreciate this difficulty by calling to mind what complex yet
symmetrical growths the cells of plants yield when inoculated by the
poison of a gall-insect. With animals various polypoid excrescences and
tumours are generally admitted[52] to be the direct product, through
proliferation, of normal cells which have become abnormal. In the
regular growth and repair of bones, the tissues undergo, as Virchow
remarks,[53] a whole series of permutations and substitutions. “The
cartilage cells may be converted by a direct transformation into
marrow-cells, and continue as such; or they may first be converted into
osseous and then into medullary tissue; or lastly, they may first be
converted into marrow and then into bone. So variable are the
permutations of these tissues, in themselves so nearly allied, and yet
in their external appearance so completely distinct.” But as these
tissues thus change their nature at any age, without any obvious change
in their nutrition, we must suppose in accordance with our hypothesis
that gemmules derived from one kind of tissue combine with the cells of
another kind, and cause the successive modifications.
We have good reason to believe that several gemmules are requisite for
the development of one and the same unit or cell; for we cannot
otherwise understand the insufficiency of a single or even of two or
three pollen-grains or spermatozoa. But we are far from knowing whether
the gemmules of all the units are free and separate from one another,
or whether some are from the first united into small aggregates. A
feather, for instance, is a complex structure, and, as each separate
part is liable to inherited variations, I conclude that each feather
generates a large number of gemmules; but it is possible that these may
be aggregated into a compound gemmule. The same remark applies to the
petals of flowers, which are sometimes highly complex structures, with
each ridge and hollow contrived for a special purpose, so that each
part must have been separately modified, and the modifications
transmitted; consequently, separate gemmules, according to our
hypothesis, must have been thrown off from each cell or unit. But, as
we sometimes see half an anther or a small portion of a filament
becoming petali-form, or parts or mere stripes of the calyx assuming
the colour and texture of the corolla, it is probable that with petals
the gemmules of each cell are not aggregated together into a compound
gemmule, but are free and separate. Even in so simple a case as that of
a perfect cell, with its protoplasmic contents, nucleus, nucleolus, and
walls, we do not know whether or not its development depends on a
compound gemmule derived from each part.[54]
Having now endeavoured to show that the several foregoing assumptions
are to a certain extent supported by analogous facts, and having
alluded to some of the most doubtful points, we will consider how far
the hypothesis brings under a single point of view the various cases
enumerated in the First Part. All the forms of reproduction graduate
into one another and agree in their product; for it is impossible to
distinguish between organisms produced from buds, from self-division,
or from fertilised germs; such organisms are liable to variations of
the same nature and to reversions of the same kind; and as, according
to our hypothesis, all the forms of reproduction depend on the
aggregation of gemmules derived from the whole body, we can understand
this remarkable agreement. Parthenogenesis is no longer wonderful, and
if we did not know that great good followed from the union of the
sexual elements derived from two distinct individuals, the wonder would
be that parthenogenesis did not occur much oftener than it does. On any
ordinary theory of reproduction the formation of graft-hybrids, and the
action of the male element on the tissues of the mother-plant, as well
as on the future progeny of female animals, are great anomalies; but
they are intelligible on our hypothesis. The reproductive organs do not
actually create the sexual elements; they merely determine the
aggregation and perhaps the multiplication of the gemmules in a special
manner. These organs, however, together with their accessory parts,
have high functions to perform. They adapt one or both elements for
independent temporary existence, and for mutual union. The stigmatic
secretion acts on the pollen of a plant of the same species in a wholly
different manner to what it does on the pollen of one belonging to a
distinct genus or family. The spermatophores of the Cephalopoda are
wonderfully complex structures, which were formerly mistaken for
parasitic worms; and the spermatozoa of some animals possess attributes
which, if observed in an independent animal, would be put down to
instinct guided by sense-organs,—as when the spermatozoa of an insect
find their way into the minute micropyle of the egg.
The antagonism which has long been observed,[55] with certain
exceptions, between growth and the power of sexual
reproduction[56]—between the repair of injuries and gemmation—and with
plants, between rapid increase by buds, rhizomes, etc., and the
production of seed, is partly explained by the gemmules not existing in
sufficient numbers for these processes to be carried on simultaneously.
Hardly any fact in physiology is more wonderful than the power of
re-growth; for instance, that a snail should be able to reproduce its
head, or a salamander its eyes, tail, and legs, exactly at the points
where they have been cut off. Such cases are explained by the presence
of gemmules derived from each part, and disseminated throughout the
body. I have heard the process compared with that of the repair of the
broken angles of a crystal by re-crystallisation; and the two processes
have this much in common, that in the one case the polarity of the
molecules is the efficient cause, and in the other the affinity of the
gemmules for particular nascent cells. But we have here to encounter
two objections which apply not only to the re-growth of a part, or of a
bisected individual, but to fissiparous generation and budding. The
first objection is that the part which is reproduced is in the same
stage of development as that of the being which has been operated on or
bisected; and in the case of buds, that the new beings thus produced
are in the same stage as that of the budding parent. Thus a mature
salamander, of which the tail has been cut off, does not reproduce a
larval tail; and a crab does not reproduce a larval leg. In the case of
budding it was shown in the first part of this chapter that the new
being thus produced does not retrograde in development,—that is, does
not pass through those earlier stages, which the fertilised germ has to
pass through. Nevertheless, the organisms operated on or multiplying
themselves by buds must, by our hypothesis, include innumerable
gemmules derived from every part or unit of the earlier stages of
development; and why do not such gemmules reproduce the amputated part
or the whole body at a corresponding early stage of development?
The second objection, which has been insisted on by Delpino, is that
the tissues, for instance, of a mature salamander or crab, of which a
limb has been removed, are already differentiated and have passed
through their whole course of development; and how can such tissues in
accordance with our hypothesis attract and combine with the gemmules of
the part which is to be reproduced? In answer to these two objections
we must bear in mind the evidence which has been advanced, showing that
at least in a large number of cases the power of re-growth is a
localised faculty, acquired for the sake of repairing special injuries
to which each particular creature is liable; and in the case of buds or
fissiparous generation, for the sake of quickly multiplying the
organism at a period of life when it can be supported in large numbers.
These considerations lead us to believe that in all such cases a stock
of nascent cells or of partially developed gemmules are retained for
this special purpose either locally or throughout the body, ready to
combine with the gemmules derived from the cells which come next in due
succession. If this be admitted we have a sufficient answer to the
above two objections. Anyhow, pangenesis seems to throw a considerable
amount of light on the wonderful power of re-growth.
It follows, also, from the view just given, that the sexual elements
differ from buds in not including nascent cells or gemmules in a
somewhat advanced stage of development, so that only the gemmules
belonging to the earliest stages are first developed. As young animals
and those which stand low in the scale generally have a much greater
capacity for re-growth than older and higher animals, it would also
appear that they retain cells in a nascent state, or partially
developed gemmules, more readily than do animals which have already
passed through a long series of developmental changes. I may here add
that although ovules can be detected in most or all female animals at
an extremely early age, there is no reason to doubt that gemmules
derived from parts modified during maturity can pass into the ovules.
With respect to hybridism, pangenesis agrees well with most of the
ascertained facts. We must believe, as previously shown, that several
gemmules are requisite for the development of each cell or unit. But
from the occurrence of parthenogenesis, more especially from those
cases in which an embryo is only partially formed, we may infer that
the female element generally includes gemmules in nearly sufficient
number for independent development, so that when united with the male
element the gemmules are superabundant. Now, when two species or races
are crossed reciprocally, the offspring do not commonly differ, and
this shows that the sexual elements agree in power, in accordance with
the view that both include the same gemmules. Hybrids and mongrels are
also generally intermediate in character between the two parent-forms,
yet occasionally they closely resemble one parent in one part and the
other parent in another part, or even in their whole structure: nor is
this difficult to understand on the admission that the gemmules in the
fertilised germ are superabundant in number, and that those derived
from one parent may have some advantage in number, affinity, or vigour
over those derived from the other parent. Crossed forms sometimes
exhibit the colour or other characters of either parent in stripes or
blotches; and this occurs in the first generation, or through reversion
in succeeding bud and seminal generations, of which fact several
instances were given in the eleventh chapter. In these cases we must
follow Naudin[57] and admit that the “essence” or “element” of the two
species,—terms which I should translate into the gemmules,—have an
affinity for their own kind, and thus separate themselves into distinct
stripes or blotches; and reasons were given, when discussing in the
fifteenth chapter the incompatibility of certain characters to unite,
for believing in such mutual affinity. When two forms are crossed, one
is not rarely found to be prepotent in the transmission of its
characters over the other; and this we can explain by again assuming
that the one form has some advantage over the other in the number,
vigour, or affinity of its gemmules. In some cases, however, certain
characters are present in the one form and latent in the other; for
instance, there is a latent tendency in all pigeons to become blue,
and, when a blue pigeon is crossed with one of any other colour, the
blue tint is generally prepotent. The explanation of this form of
prepotency will be obvious when we come to the consideration of
Reversion.
When two distinct species are crossed, it is notorious that they do not
yield the full or proper number of offspring; and we can only say on
this head that, as the development of each organism depends on such
nicely-balanced affinities between a host of gemmules and nascent
cells, we need not feel at all surprised that the commixture of
gemmules derived from two distinct species should lead to partial or
complete failure of development. With respect to the sterility of
hybrids produced from the union of two distinct species, it was shown
in the nineteenth chapter that this depends exclusively on the
reproductive organs being specially affected; but why these organs
should be thus affected we do not know, any more than why unnatural
conditions of life, though compatible with health, should cause
sterility; or why continued close interbreeding, or the illegitimate
unions of heterostyled plants, induce the same result. The conclusion
that the reproductive organs alone are affected, and not the whole
organisation, agrees perfectly with the unimpaired or even increased
capacity in hybrid plants for propagation by buds; for this implies,
according to our hypothesis, that the cells of the hybrids throw off
hybridised gemmules, which become aggregated into buds, but fail to
become aggregated within the reproductive organs, so as to form the
sexual elements. In a similar manner many plants, when placed under
unnatural conditions, fail to produce seed, but can readily be
propagated by buds. We shall presently see that pangenesis agrees well
with the strong tendency to reversion exhibited by all crossed animals
and plants.
Each organism reaches maturity through a longer or shorter course of
growth and development: the former term being confined to mere increase
of size, and development to changed structure. The changes may be small
and insensibly slow, as when a child grows into a man, or many, abrupt,
and slight, as in the metamorphoses of certain ephemerous insects, or,
again, few and strongly-marked, as with most other insects. Each newly
formed part may be moulded within a previously existing and
corresponding part, and in this case it will appear, falsely as I
believe, to be developed from the old part; or it may be formed within
a distinct part of the body, as in the extreme cases of metagenesis. An
eye, for instance, may be developed at a spot where no eye previously
existed. We have also seen that allied organic beings in the course of
their metamorphoses sometimes attain nearly the same structure after
passing through widely different forms; or conversely, after passing
through nearly the same early forms, arrive at widely different mature
forms. In these cases it is very difficult to accept the common view
that the first-formed cells or units possess the inherent power,
independently of any external agency, of producing new structures
wholly different in form, position, and function. But all these cases
become plain on the hypothesis of pangenesis. The units, during each
stage of development, throw off gemmules, which, multiplying, are
transmitted to the offspring. In the offspring, as soon as any
particular cell or unit becomes partially developed, it unites with
(or, to speak metaphorically, is fertilised by) the gemmule of the next
succeeding cell, and so onwards. But organisms have often been
subjected to changed conditions of life at a certain stage of their
development, and in consequence have been slightly modified; and the
gemmules cast off from such modified parts will tend to reproduce parts
modified in the same manner. This process may be repeated until the
structure of the part becomes greatly changed at one particular stage
of development, but this will not necessarily affect other parts,
whether previously or subsequently formed. In this manner we can
understand the remarkable independence of structure in the successive
metamorphoses, and especially in the successive metageneses of many
animals. In the case, however, of diseases which supervene during old
age, subsequently to the ordinary period of procreation, and which,
nevertheless, are sometimes inherited, as occurs with brain and heart
complaints, we must suppose that the organs were affected at an early
age and threw off at this period affected gemmules; but that the
affection became visible or injurious only after the prolonged growth,
in the strict sense of the word, of the part. In all the changes of
structure which regularly supervene during old age, we probably see the
effects of deteriorated growth, and not of true development.
The principle of the independent formation of each part, owing to the
union of the proper gemmules with certain nascent cells, together with
the superabundance of the gemmules derived from both parents, and the
subsequent self-multiplication of the gemmules, throws light on a
widely different group of facts, which on any ordinary view of
development appears very strange. I allude to organs which are
abnormally transposed or multiplied. For instance, a curious case has
been recorded by Dr. Elliott Coues[58] of a monstrous chicken with a
perfect additional _right_ leg articulated to the _left_ side of the
pelvis. Gold-fish often have supernumerary fins placed on various parts
of their bodies. When the tail of a lizard is broken off, a double tail
is sometimes reproduced; and when the foot of the salamander was
divided longitudinally by Bonnet, additional digits were occasionally
formed. Valentin injured the caudal extremity of an embryo, and three
days afterwards it produced rudiments of a double pelvis and of double
hind-limbs.[59] When frogs, toads, etc., are born with their limbs
doubled, as sometimes happens, the doubling, as Gervais remarks,[60]
cannot be due to the complete fusion of two embryos, with the exception
of the limbs, for the larvæ are limbless. The same argument is
applicable[61] to certain insects produced with multiple legs or
antennæ, for these are metamorphosed from apodal or antennæ-less larvæ.
Alphonse Milne-Edwards[62] has described the curious case of a
crustacean in which one eye-peduncle supported, instead of a complete
eye, only an imperfect cornea, and out of the centre of this a portion
of an antenna was developed. A case has been recorded[63] of a man who
had during both dentitions a double tooth in place of the left second
incisor, and he inherited this peculiarity from his paternal
grandfather. Several cases are known[64] of additional teeth having
been developed in the orbit of the eye, and, more especially with
horses, in the palate. Hairs occasionally appear in strange situations,
as “within the substance of the brain.”[65] Certain breeds of sheep
bear a whole crowd of horns on their foreheads. As many as five spurs
have been seen on both legs of certain Game-fowls. In the Polish fowl
the male is ornamented with a topknot of hackles like those on his
neck, whilst the female has a top-knot formed of common feathers. In
feather-footed pigeons and fowls, feathers like those on the wing arise
from the outer side of the legs and toes. Even the elemental parts of
the same feather may be transposed; for in the Sebastopol goose,
barbules are developed on the divided filaments of the shaft. Imperfect
nails sometimes appear on the stumps of the amputated fingers of
man[66] and it is an interesting fact that with the snake-like
Saurians, which present a series with more and more imperfect limbs,
the terminations of the phalanges first disappear, “the nails becoming
transferred to their proximal remnants, or even to parts which are not
phalanges.”[67]
Analogous cases are of such frequent occurrence with plants that they
do not strike us with sufficient surprise. Supernumerary petals,
stamens, and pistils, are often produced. I have seen a leaflet low
down in the compound leaf of _Vicia sativa_ replaced by a tendril; and
a tendril possesses many peculiar properties, such as spontaneous
movement and irritability. The calyx sometimes assumes, either wholly
or by stripes, the colour and texture of the corolla. Stamens are so
frequently converted into petals, more or less completely, that such
cases are passed over as not deserving notice; but as petals have
special functions to perform, namely, to protect the included organs,
to attract insects, and in not a few cases to guide their entrance by
well-adapted contrivances, we can hardly account for the conversion of
stamens into petals merely by unnatural or superfluous nourishment.
Again, the edge of a petal may occasionally be found including one of
the highest products of the plant, namely, pollen; for instance, I have
seen the pollen-mass of an Ophrys, which is a very complex structure,
developed in the edge of an upper petal. The segments of the calyx of
the common pea have been observed partially converted into carpels,
including ovules, and with their tips converted into stigmas. Mr.
Salter and Dr. Maxwell Masters have found pollen within the ovules of
the passion-flower and of the rose. Buds may be developed in the most
unnatural positions, as on the petal of a flower. Numerous analogous
facts could be given.[68]
I do not know how physiologists look at such facts as the foregoing.
According to the doctrine of pangenesis, the gemmules of the transposed
organs become developed in the wrong place, from uniting with wrong
cells or aggregates of cells during their nascent state; and this would
follow from a slight modification in their elective affinities. Nor
ought we to feel much surprise at the affinities of cells and gemmules
varying, when we remember the many curious cases given in the
seventeenth chapter, of plants which absolutely refuse to be fertilised
by their own pollen, though abundantly fertile with that of any other
individual of the same species, and in some cases only with that of a
distinct species. It is manifest that the sexual elective affinities of
such plants—to use the term employed by Gärtner—have been modified. As
the cells of adjoining or homologous parts will have nearly the same
nature, they will be particularly liable to acquire by variation each
other’s elective affinities; and we can thus understand to a certain
extent such cases as a crowd of horns on the heads of certain sheep, of
several spurs on the legs of fowls, hackle-like feathers on the heads
of the males of other fowls, and with the pigeon wing-like feathers on
their legs and membrane between their toes, for the leg is the
homologue of the wing. As all the organs of plants are homologous and
spring from a common axis, it is natural that they should be eminently
liable to transposition. It ought to be observed that when any compound
part, such as an additional limb or an antenna, springs from a false
position, it is only necessary that the few first gemmules should be
wrongly attached; for these whilst developing would attract other
gemmules in due succession, as in the re-growth of an amputated limb.
When parts which are homologous and similar in structure, as the
vertebræ of snakes or the stamens of polyandrous flowers, etc., are
repeated many times in the same organism, closely allied gemmules must
be extremely numerous, as well as the points to which they ought to
become united; and, in accordance with the foregoing views, we can to a
certain extent understand Isid. Geoffroy Saint-Hilaire’s law, that
parts, which are already multiple, are extremely liable to vary in
number.
Variability often depends, as I have attempted to show, on the
reproductive organs being injuriously affected by changed conditions;
and in this case the gemmules derived from the various parts of the
body are probably aggregated in an irregular manner, some superfluous
and others deficient. Whether a superabundance of gemmules would lead
to the increased size of any part cannot be told; but we can see that
their partial deficiency, without necessarily leading to the entire
abortion of the part, might cause considerable modifications; for in
the same manner as plants, if their own pollen be excluded, are easily
hybridised, so, in the case of cells, if the properly succeeding
gemmules were absent, they would probably combine easily with other and
allied gemmules, as we have just seen with transposed parts.
In variations caused by the direct action of changed conditions, of
which several instances have been given, certain parts of the body are
directly affected by the new conditions, and consequently throw off
modified gemmules, which are transmitted to the offspring. On any
ordinary view it is unintelligible how changed conditions, whether
acting on the embryo, the young or the adult, can cause inherited
modifications. It is equally or even more unintelligible on any
ordinary view, how the effects of the long-continued use or disuse of a
part, or of changed habits of body or mind, can be inherited. A more
perplexing problem can hardly be proposed; but on our view we have only
to suppose that certain cells become at last structurally modified; and
that these throw off similarly modified gemmules. This may occur at any
period of development, and the modification will be inherited at a
corresponding period; for the modified gemmules will unite in all
ordinary cases with the proper preceding cells, and will consequently
be developed at the same period at which the modification first arose.
With respect to mental habits or instincts, we are so profoundly
ignorant of the relation between the brain and the power of thought
that we do not know positively whether a fixed habit induces any change
in the nervous system, though this seems highly probable; but when such
habit or other mental attribute, or insanity, is inherited, we must
believe that some actual modification is transmitted;[69] and this
implies, according to our hypothesis, that gemmules derived from
modified nerve-cells are transmitted to the offspring.
It is generally necessary that an organism should be exposed during
several generations to changed conditions or habits, in order that any
modification thus acquired should appear in the offspring. This may be
partly due to the changes not being at first marked enough to catch
attention, but this explanation is insufficient; and I can account for
the fact only by the assumption, which we shall see under the head of
reversion is strongly supported, that gemmules derived from each
unmodified unit or part are transmitted in large numbers to successive
generations, and that the gemmules derived from the same unit after it
has been modified go on multiplying under the same favourable
conditions which first caused the modification, until at last they
become sufficiently numerous to overpower and supplant the old
gemmules.
A difficulty may be here noticed; we have seen that there is an
important difference in the frequency, though not in the nature, of the
variations in plants propagated by sexual and asexual generation. As
far as variability depends on the imperfect action of the reproductive
organs under changed conditions, we can at once see why plants
propagated asexually should be far less variable than those propagated
sexually. With respect to the direct action of changed conditions, we
know that organisms produced from buds do not pass through the earlier
phases of development; they will therefore not be exposed, at that
period of life when structure is most readily modified, to the various
causes inducing variability in the same manner as are embryos and young
larval forms; but whether this is a sufficient explanation I know not.
With respect to variations due to reversion, there is a similar
difference between plants propagated from buds and seeds. Many
varieties can be propagated securely by buds, but generally or
invariably revert to their parent-forms by seed. So, also, hybridised
plants can be multiplied to any extent by buds, but are continually
liable to reversion by seed,—that is, to the loss of their hybrid or
intermediate character. I can offer no satisfactory explanation of
these facts. Plants with variegated leaves, phloxes with striped
flowers, barberries with seedless fruit, can all be securely propagated
by buds taken from the stem or branches; but buds from the roots of
these plants almost invariably lose their character and revert to their
former condition. This latter fact is also inexplicable, unless buds
developed from the roots are as distinct from those on the stem, as is
one bud on the stem from another, and we know that these latter behave
like independent organisms.
Finally, we see that on the hypothesis of pangenesis variability
depends on at least two distinct groups of causes. Firstly, the
deficiency, superabundance, and transposition of gemmules, and the
redevelopment of those which have long been dormant; the gemmules
themselves not having undergone any modification; and such changes will
amply account for much fluctuating variability. Secondly, the direct
action of changed conditions on the organisation, and of the increased
use or disuse of parts; and in this case the gemmules from the modified
units will be themselves modified, and, when sufficiently multiplied,
will supplant the old gemmules and be developed into new structures.
Turning now to the laws of Inheritance. If we suppose a homogeneous
gelatinous protozoon to vary and assume a reddish colour, a minute
separated particle would naturally, as it grew to full size, retain the
same colour; and we should have the simplest form of inheritance.[70]
Precisely the same view may be extended to the infinitely numerous and
diversified units of which the whole body of one of the higher animals
is composed; the separated particles being our gemmules. We have
already sufficiently discussed by implication, the important principle
of inheritance at corresponding ages. Inheritance as limited by sex and
by the season of the year (for instance with animals becoming white in
winter) is intelligible if we may believe that the elective affinities
of the units of the body are slightly different in the two sexes,
especially at maturity, and in one or both sexes at different seasons,
so that they unite with different gemmules. It should be remembered
that, in the discussion on the abnormal transposition of organs, we
have seen reason to believe that such elective affinities are readily
modified. But I shall soon have to recur to sexual and seasonal
inheritance. These several laws are therefore explicable to a large
extent through pangenesis, and on no other hypothesis which has as yet
been advanced.
But it appears at first sight a fatal objection to our hypothesis that
a part or organ may be removed during several successive generations,
and if the operation be not followed by disease, the lost part
reappears in the offspring. Dogs and horses formerly had their tails
docked during many generations without any inherited effect; although,
as we have seen, there is some reason to believe that the tailless
condition of certain sheep-dogs is due to such inheritance.
Circumcision has been practised by the Jews from a remote period, and
in most cases the effects of the operation are not visible in the
offspring; though some maintain that an inherited effect does
occasionally appear. If inheritance depends on the presence of
disseminated gemmules derived from all the units of the body, why does
not the amputation or mutilation of a part, especially if effected on
both sexes, invariably affect the offspring? The answer in accordance
with our hypothesis probably is that gemmules multiply and are
transmitted during a long series of generations—as we see in the
reappearance of zebrine stripes on the horse—in the reappearance of
muscles and other structures in man which are proper to his lowly
organised progenitors, and in many other such cases. Therefore the
long-continued inheritance of a part which has been removed during many
generations is no real anomaly, for gemmules formerly derived from the
part are multiplied and transmitted from generation to generation.
We have as yet spoken only of the removal of parts, when not followed
by morbid action: but when the operation is thus followed, it is
certain that the deficiency is sometimes inherited. In a former chapter
instances were given, as of a cow, the loss of whose horn was followed
by suppuration, and her calves were destitute of a horn on the same
side of their heads. But the evidence which admits of no doubt is that
given by Brown-Séquard with respect to guinea-pigs, which after their
sciatic nerves had been divided, gnawed off their own gangrenous toes,
and the toes of their offspring were deficient in at least thirteen
instances on the corresponding feet. The inheritance of the lost part
in several of these cases is all the more remarkable as only one parent
was affected; but we know that a congenital deficiency is often
transmitted from one parent alone—for instance, the offspring of
hornless cattle of either sex, when crossed with perfect animals, are
often hornless. How, then, in accordance with our hypothesis can we
account for mutilations being sometimes strongly inherited, if they are
followed by diseased action? The answer probably is that all the
gemmules of the mutilated or amputated part are gradually attracted to
the diseased surface during the reparative process, and are there
destroyed by the morbid action.
A few words must be added on the complete abortion of organs. When a
part becomes diminished by disuse prolonged during many generations,
the principle of economy of growth, together with intercrossing, will
tend to reduce it still further as previously explained, but this will
not account for the complete or almost complete obliteration of, for
instance, a minute papilla of cellular tissue representing a pistil, or
of a microscopically minute nodule of bone representing a tooth. In
certain cases of suppression not yet completed, in which a rudiment
occasionally reappears through reversion, dispersed gemmules derived
from this part must, according to our view, still exist; we must
therefore suppose that the cells, in union with which the rudiment was
formerly developed, fail in their affinity for such gemmules, except in
the occasional cases of reversion. But when the abortion is complete
and final, the gemmules themselves no doubt perish; nor is this in any
way improbable, for, though a vast number of active and long-dormant
gemmules are nourished in each living creature, yet there must be some
limit to their number; and it appears natural that gemmules derived
from reduced and useless parts would be more liable to perish than
those freshly derived from other parts which are still in full
functional activity.
The last subject that need be discussed, namely, Reversion, rests on
the principle that transmission and development, though generally
acting in conjunction, are distinct powers; and the transmission of
gemmules with their subsequent development shows us how this is
possible. We plainly see the distinction in the many cases in which a
grandfather transmits to his grandson, through his daughter, characters
which she does not, or cannot, possess. But before proceeding, it will
be advisable to say a few words about latent or dormant characters.
Most, or perhaps all, of the secondary characters, which appertain to
one sex, lie dormant in the other sex; that is, gemmules capable of
development into the secondary male sexual characters are included
within the female; and conversely female characters in the male: we
have evidence of this in certain masculine characters, both corporeal
and mental, appearing in the female, when her ovaria are diseased or
when they fail to act from old age. In like manner female characters
appear in castrated males, as in the shape of the horns of the ox, and
in the absence of horns in castrated stags. Even a slight change in the
conditions of life due to confinement sometimes suffices to prevent the
development of masculine characters in male animals, although their
reproductive organs are not permanently injured. In the many cases in
which masculine characters are periodically renewed, these are latent
at other seasons; inheritance as limited by sex and season being here
combined. Again, masculine characters generally lie dormant in male
animals until they arrive at the proper age for reproduction. The
curious case formerly given of a Hen which assumed the masculine
characters, not of her own breed but of a remote progenitor,
illustrates the close connection between latent sexual characters and
ordinary reversion.
With those animals and plants which habitually produce several forms,
as with certain butterflies described by Mr. Wallace, in which three
female forms and one male form co-exist, or, as with the trimorphic
species of Lythrum and Oxalis, gemmules capable of reproducing these
different forms must be latent in each individual.
Insects are occasionally produced with one side or one quarter of their
bodies like that of the male, with the other half or three-quarters
like that of the female. In such cases the two sides are sometimes
wonderfully different in structure, and are separated from each other
by a sharp line. As gemmules derived from every part are present in
each individual of both sexes, it must be the elective affinities of
the nascent cells which in these cases differ abnormally on the two
sides of the body. Almost the same principle comes into play with those
animals, for instance, certain gasteropods and Verruca amongst
cirripedes, which normally have the two sides of the body constructed
on a very different plan; and yet a nearly equal number of individuals
have either side modified in the same remarkable manner.
Reversion, in the ordinary sense of the word, acts so incessantly, that
it evidently forms an essential part of the general law of inheritance.
It occurs with beings, however propagated, whether by buds or seminal
generation, and sometimes may be observed with advancing age even in
the same individual. The tendency to reversion is often induced by a
change of conditions, and in the plainest manner by crossing. Crossed
forms of the first generation are generally nearly intermediate in
character between their two parents; but in the next generation the
offspring commonly revert to one or both of their grandparents, and
occasionally to more remote ancestors. How can we account for these
facts? Each unit in a hybrid must throw off, according to the doctrine
of pangenesis, an abundance of hybridised gemmules, for crossed plants
can be readily and largely propagated by buds; but by the same
hypothesis dormant gemmules derived from both pure parent-forms are
likewise present; and as these gemmules retain their normal condition,
they would, it is probable, be enabled to multiply largely during the
lifetime of each hybrid. Consequently the sexual elements of a hybrid
will include both pure and hybridised gemmules; and when two hybrids
pair, the combination of pure gemmules derived from the one hybrid with
the pure gemmules of the same parts derived from the other, would
necessarily lead to complete reversion of character; and it is,
perhaps, not too bold a supposition that unmodified and undeteriorated
gemmules of the same nature would be especially apt to combine. Pure
gemmules in combination with hybridised gemmules would lead to partial
reversion. And lastly, hybridised gemmules derived from both
parent-hybrids would simply reproduce the original hybrid form.[71] All
these cases and degrees of reversion incessantly occur.
It was shown in the fifteenth chapter that certain characters are
antagonistic to each other or do not readily blend; hence, when two
animals with antagonistic characters are crossed, it might well happen
that a sufficiency of gemmules in the male alone for the reproduction
of his peculiar characters, and in the female alone for the
reproduction of her peculiar characters, would not be present; and in
this case dormant gemmules derived from the same part in some remote
progenitor might easily gain the ascendancy, and cause the reappearance
of the long-lost character. For instance, when black and white pigeons,
or black and white fowls, are crossed,—colours which do not readily
blend,—blue plumage in the one case, evidently derived from the
rock-pigeon, and red plumage in the other case, derived from the wild
jungle-cock, occasionally reappear. With uncrossed breeds the same
result follows, under conditions which favour the multiplication and
development of certain dormant gemmules, as when animals become feral
and revert to their pristine character. A certain number of gemmules
being requisite for the development of each character, as is known to
be the case from several spermatozoa or pollen-grains being necessary
for fertilisation, and time favouring their multiplication, will
perhaps account for the curious cases, insisted on by Mr. Sedgwick, of
certain diseases which regularly appear in alternate generations. This
likewise holds good, more or less strictly, with other weakly inherited
modifications. Hence, as I have heard it remarked, certain diseases
appear to gain strength by the intermission of a generation. The
transmission of dormant gemmules during many successive generations is
hardly in itself more improbable, as previously remarked, than the
retention during many ages of rudimentary organs, or even only of a
tendency to the production of a rudiment; but there is no reason to
suppose that dormant gemmules can be transmitted and propagated for
ever. Excessively minute and numerous as they are believed to be, an
infinite number derived, during a long course of modification and
descent, from each unit of each progenitor, could not be supported or
nourished by the organism. But it does not seem improbable that certain
gemmules, under favourable conditions, should be retained and go on
multiplying for a much longer period than others. Finally, on the view
here given, we certainly gain some insight into the wonderful fact that
the child may depart from the type of both its parents, and resemble
its grandparents, or ancestors removed by many hundreds of generations.
_Conclusion._
The hypothesis of Pangenesis, as applied to the several great classes
of facts just discussed, no doubt is extremely complex, but so are the
facts. The chief assumption is that all the units of the body, besides
having the universally admitted power of growing by self-division,
throw off minute gemmules which are dispersed through the system. Nor
can this assumption be considered as too bold, for we know from the
cases of graft-hybridisation that formative matter of some kind is
present in the tissues of plants, which is capable of combining with
that included in another individual, and of reproducing every unit of
the whole organism. But we have further to assume that the gemmules
grow, multiply, and aggregate themselves into buds and the sexual
elements; their development depending on their union with other nascent
cells or units. They are also believed to be capable of transmission in
a dormant state, like seeds in the ground, to successive generations.
In a highly-organised animal, the gemmules thrown off from each
different unit throughout the body must be inconceivably numerous and
minute. Each unit of each part, as it changes during development, and
we know that some insects undergo at least twenty metamorphoses, must
throw off its gemmules. But the same cells may long continue to
increase by self-division, and even become modified by absorbing
peculiar nutriment, without necessarily throwing off modified gemmules.
All organic beings, moreover, include many dormant gemmules derived
from their grandparents and more remote progenitors, but not from all
their progenitors. These almost infinitely numerous and minute gemmules
are contained within each bud, ovule, spermatozoon, and pollen-grain.
Such an admission will be declared impossible; but number and size are
only relative difficulties. Independent organisms exist which are
barely visible under the highest powers of the microscope, and their
germs must be excessively minute. Particles of infectious matter, so
small as to be wafted by the wind or to adhere to smooth paper, will
multiply so rapidly as to infect within a short time the whole body of
a large animal. We should also reflect on the admitted number and
minuteness of the molecules composing a particle of ordinary matter.
The difficulty, therefore, which at first appears insurmountable, of
believing in the existence of gemmules so numerous and small as they
must be according to our hypothesis, has no great weight.
The units of the body are generally admitted by physiologists to be
autonomous. I go one step further and assume that they throw off
reproductive gemmules. Thus an organism does not generate its kind as a
whole, but each separate unit generates its kind. It has often been
said by naturalists that each cell of a plant has the potential
capacity of reproducing the whole plant; but it has this power only in
virtue of containing gemmules derived from every part. When a cell or
unit is from some cause modified, the gemmules derived from it will be
in like manner modified. If our hypothesis be provisionally accepted,
we must look at all the forms of asexual reproduction, whether
occurring at maturity or during youth, as fundamentally the same, and
dependent on the mutual aggregation and multiplication of the gemmules.
The re-growth of an amputated limb and the healing of a wound is the
same process partially carried out. Buds apparently include nascent
cells, belonging to that stage of development at which the budding
occurs, and these cells are ready to unite with the gemmules derived
from the next succeeding cells. The sexual elements, on the other hand,
do not include such nascent cells; and the male and female elements
taken separately do not contain a sufficient number of gemmules for
independent development, except in the cases of parthenogenesis. The
development of each being, including all the forms of metamorphosis and
metagenesis, depends on the presence of gemmules thrown off at each
period of life, and on their development, at a corresponding period, in
union with preceding cells. Such cells may be said to be fertilised by
the gemmules which come next in due order of development. Thus the act
of ordinary impregnation and the development of each part in each being
are closely analogous processes. The child, strictly speaking, does not
grow into the man, but includes germs which slowly and successively
become developed and form the man. In the child, as well as in the
adult, each part generates the same part. Inheritance must be looked at
as merely a form of growth, like the self-division of a lowly-organised
unicellular organism. Reversion depends on the transmission from the
forefather to his descendants of dormant gemmules, which occasionally
become developed under certain known or unknown conditions. Each animal
and plant may be compared with a bed of soil full of seeds, some of
which soon germinate, some lie dormant for a period, whilst others
perish. When we hear it said that a man carries in his constitution the
seeds of an inherited disease, there is much truth in the expression.
No other attempt, as far as I am aware, has been made, imperfect as
this confessedly is, to connect under one point of view these several
grand classes of facts. An organic being is a microcosm—a little
universe, formed of a host of self-propagating organisms, inconceivably
minute and numerous as the stars in heaven.
REFERENCES
[1] This hypothesis has been severely criticised by many writers, and
it will be fair to give references to the more important articles. The
best essay which I have seen is by Prof. Delpino, entitled ‘Sulla
Darwiniana Teoria della Pangenesi, 1869,’ of which a translation
appeared in ‘Scientific Opinion,’ Sept. 29th, 1869, and the succeeding
numbers. He rejects the hypothesis, but criticises it fairly, and I
have found his criticisms very useful. Mr. Mivart (‘Genesis of
Species,’ 1871, chap. x.) follows Delpino, but adds no new objections
of any weight. Dr. Bastian (‘The Beginnings of Life,’ 1872, vol. ii.
p. 98) says that the hypothesis “looks like a relic of the old rather
than a fitting appanage of the new evolution philosophy.” He shows
that I ought not to have used the term “pangenesis,” as it had been
previously used by Dr. Gros in another sense. Dr. Lionel Beale
(‘Nature,’ May 11th, 1871, p. 26) sneers at the whole doctrine with
much acerbity and some justice. Prof. Wigand (‘Schriften der Gesell.
der gesammt. Naturwissen. zu Marburg,’ B. ix. 1870) considers the
hypothesis as unscientific and worthless. Mr. G. H. Lewes
(‘Fortnightly Review,’ Nov. 1st, 1868, p. 503) seems to consider that
it may be useful: he makes many good criticisms in a perfectly fair
spirit. Mr. F. Galton, after describing his valuable experiments
(‘Proc. Royal Soc.,’ vol. xix. p. 393) on the intertransfusion of the
blood of distinct varieties of the rabbit, concludes by saying that in
his opinion the results negative beyond all doubt the doctrine of
Pangenesis. He informs me that subsequently to the publication of his
paper he continued his experiments on a still larger scale for two
more generations, without any sign of mongrelism showing itself in the
very numerous offspring. I certainly should have expected that
gemmules would have been present in the blood, but this is no
necessary part of the hypothesis, which manifestly applies to plants
and the lowest animals. Mr. Galton, in a letter to ‘Nature’ (April
27th, 1871, p. 502), also criticises various incorrect expressions
used by me. On the other hand, several writers have spoken favourably
of the hypothesis, but there would be no use in giving references to
their articles. I may, however, refer to Dr. Ross’ work, ‘The Graft
Theory of Disease; being an application of Mr. Darwin’s hypothesis of
Pangenesis,’ 1872, as he gives several original and ingenious
discussions.
[2] Quoted by Paget, ‘Lectures on Pathology,’ 1853, p. 159.
[3] Dr. Lachmann, also, observes (‘Annals and Mag. of Nat. History,’
2nd series, vol. xix. 1857, p. 231) with respect to infusoria, that
“fissation and gemmation pass into each other almost imperceptibly.”
Again, Mr. W. C. Minor (‘Annals and Mag. of Nat. Hist.,’ 3rd series,
vol. xi. p. 328) shows that with Annelids the distinction that has
been made between fission and budding is not a fundamental one. _See
also_ Professor Clark’s work ‘Mind in Nature,’ New York, 1865, pp. 62,
94.
[4] _See_ Bonnet, ‘Œuvres d’Hist. Nat.,’ tom. v., 1781, p. 339, for
remarks on the budding-out of the amputated limbs of Salamanders.
[5] Paget, ‘Lectures on Pathology,’ 1853, p. 158.
[6] Ibid., pp. 152, 164.
[7] Translated in ‘Annals and Mag. of Nat. Hist.,’ April 1870, p. 272.
[8] Bischoff, as quoted by von Siebold, “Ueber Parthenogenesis,”
‘Sitzung der math. phys. Classe.’ Munich, Nov. 4th, 1871, p. 240. _See
also_ Quatrefages, ‘Annales des Sc. Nat. Zoolog.,’ 3rd series, 1850,
p. 138.
[9] ‘On the Asexual Reproduction of Cecidomyide Larvæ,’ translated in
‘Annals and Mag. of Nat. Hist.,’ March 1866, pp. 167, 171.
[10] Prof. Allman speaks (‘Transact. R. Soc. of Edinburgh,’ vol.
xxvi., 1870, p. 102) decisively on this head with respect to the
Hydroida: he says, “It is a universal law in the succession of zooids,
that no retrogression ever takes place in the series.”
[11] ‘Annals and Mag. of Nat. Hist.,’ 2nd series, vol. xx., 1857, pp.
153-455.
[12] ‘Annales des Sc. Nat.,’ 3rd series, 1850, tom. xiii.
[13] ‘Transact. Phil. Soc.,’ 1851, pp. 196, 208, 210; 1853 pp. 245,
247.
[14] ‘Beitrage zur Kenntniss,’ etc., 1844, s. 345.
[15] ‘Nouvelles Archives du Muséum,’ tom. i. p. 27.
[16] As quoted by Sir J. Lubbock in ‘Nat. Hist. Review,’ 1862, p. 345.
Weijenbergh also raised (‘Nature,’ Dec. 21st, 1871, p. 149) two
successive generations from unimpregnated females of another
lepidopterous insect, _Liparis dispar._ These females did not produce
at most one-twentieth of their full complement of eggs, and many of
the eggs were worthless. Moreover the caterpillars raised from these
unfertilised eggs “possessed far less vitality” than those from
fertilised eggs. In the third parthenogenetic generation not a single
egg yielded a caterpillar.
[17] ‘Entwickelungsgeschichte der Siphonophora,’ 1869, p. 73.
[18] Spallanzani, ‘An Essay on Animal Reproduction,’ translated by Dr.
Maty, 1769, p. 79. Bonnet, ‘Œuvres d’Hist. Nat.,’ tom. v., part i.,
4to. edit., 1781, pp. 343, 350.
[19] Vulpian, as quoted by Prof. Faivre, ‘La Variabilité des Espèces,’
1868, p. 112.
[20] Dr. P. Hoy, ‘The American Naturalist,’ Sept. 1871, p. 579.
[21] Dr. Gunther, in Owen’s ‘Anatomy of Vertebrates,’ vol. i., 1866,
p. 567. Spallanzani has made similar observations.
[22] A thrush was exhibited before the British Association at Hull in
1853 which had lost its tarsus, and this member, it was asserted, had
been thrice reproduced; having been lost, I presume, each time by
disease. Sir J. Paget informs me that he feels some doubt about the
facts recorded by Sir J. Simpson (‘Monthly Journal of Medical
Science,’ Edinburgh, 1848, new series, vol. ii., p. 890) of the
re-growth of limbs in the womb in the case of man.
[23] ‘Atti della Soc. Ital. di Sc. Nat.,’ vol. xi., 1869, p. 493.
[24] Lessona states that this is so in the paper just referred to.
_See also_ ‘The American Naturalist,’ Sept. 1871, p. 579.
[25] ‘Comptes Rendus,’ Oct. 1st, 1866, and June, 1867.
[26] Bonnet, ‘Oeuvres Hist. Nat.,’ vol. v., p. 294, as quoted by Prof.
Rolleston in his remarkable address to the 36th annual meeting of the
British Medical Association.
[27] ‘Proc. Boston Soc. of Nat. Hist.,’ vol. xii., 1868-69, p. 1.
[28] ‘Transact. Linn. Soc.,’ vol. xxiv., 1863, p. 62.
[29] ‘Parthenogenesis,’ 1849, pp. 25, 26. Prof. Huxley has some
excellent remarks (‘Medical Times,’ 1856, p. 637) on this subject in
reference to the development of star-fishes, and shows how curiously
metamorphosis graduates into gemmation or zoid-formation, which is in
fact the same as metagenesis.
[30] Prof. J. Reay Greene, in Günther’s ‘Record of Zoolog. Lit.,’
1865, p. 625.
[31] Fritz Müller, ‘Für Darwin,’ 1864, s. 65, 71. The highest
authority on crustaceans, Prof. Milne-Edwards, insists (‘Annal. des
Sci. Nat.,’ 2nd series, Zoolog., tom. iii., p. 322) on the difference
in the metamorphosis of closely-allied genera.
[32] Prof. Allman, in ‘Annals and Mag. of Nat. Hist.,’ 3rd series,
vol. xiii., 1864, p. 348; Dr. S. Wright, ibid., vol. viii., 1861, p.
127. _See also_ p. 358 for analogous statements by Sars.
[33] ‘Tissus Vivants,’ 1866, p. 22.
[34] ‘Cellular Pathology,’ translated by Dr. Chance, 1860, pp. 14, 18,
83, 460.
[35] Paget, ‘Surgical Pathology,’ vol. i., 1853, pp. 12-14.
[36] Ibid., p. 19.
[37] _See_ Prof. Mantegazza’s interesting work, ‘Degli innesti
Animali,’ etc., Milano, 1865, p. 51, tab. 3.
[38] ‘De la Production Artificielle des Os,’ p. 8.
[39] Isidore Geoffroy Saint-Hilaire, ‘Hist. des Anomalies,’ tom. ii.,
pp. 549, 560, 562; Virchow, ibid., p. 484. Lawson Tait, ‘The Pathology
of Diseases of the Ovaries,’ 1874, pp. 61, 62.
[40] For the most recent classification of cells, _see_ Ernst Hackel,
‘Generelle Morpholog.,’ B. ii., 1866, s. 275.
[41] Dr. W. Turner, ‘The Present Aspect of Cellular Pathology,’
‘Edinburgh Medical Journal,’ April 1863.
[42] Mr. G. H. Lewes (‘Fortnightly Review,’ Nov. 1st, 1868, p. 506)
remarks on the number of writers who have advanced nearly similar
views. More than two thousand years ago Aristotle combated a view of
this kind, which, as I hear from Dr. W. Ogle, was held by Hippocrates
and others. Ray, in his ‘Wisdom of God’ (2nd edit., 1692, p. 68), says
that “every part of the body seems to club and contribute to the
seed.” The “organic molecules” of Buffon (‘Hist. Nat. Gen.,’ edit. of
1749, tom. ii., pp. 54, 62, 329, 333, 420, 425) appear at first sight
to be the same as the gemmules of my hypothesis, but they are
essentially different. Bonnet (‘Œuvres d’Hist. Nat.,’ tom. v., part
i., 1781, 4to edit., p. 334) speaks of the limbs having germs adapted
for the reparation of all possible losses; but whether these germs are
supposed to be the same with those within buds and the sexual organs
is not clear. Prof. Owen says (‘Anatomy of Vertebrates,’ vol. iii.,
1868, p. 813) that he fails to see any fundamental difference between
the views which he propounded in his ‘Parthenogenesis’ (1849, pp.
5-8), and which he now considers as erroneous, and my hypothesis of
pangenesis: but a reviewer (‘Journal of Anat. and Phys.,’ May 1869, p.
441) shows how different they really are. I formerly thought that the
“physiological units” of Herbert Spencer (‘Principles of Biology,’
vol. i., chaps. iv. and viii., 1863-64) were the same as my gemmules,
but I now know that this is not the case. Lastly, it appears from a
review of the present work by Prof. Mantegazza (‘Nuova Antologia,
Maggio,’ 1868), that he (in his ‘Elementi di Igiene,’ Ediz. iii., p.
540) clearly foresaw the doctrine of pangenesis.
[43] Mr. Lowne has observed (‘Journal of Queckett Microscopical Club,’
Sept. 23rd, 1870) certain remarkable changes in the tissues of the
larva of a fly, which makes him believe “it possible that organs and
organisms are sometimes developed by the aggregation of excessively
minute gemmules, such as those which Mr. Darwin’s hypothesis demands.”
[44] ‘Annales des Sc. Nat.,’ 3rd series, Bot., tom. xiv., 1850, p.
244.
[45] ‘Disease Germs,’ p. 20.
[46] _See_ some very interesting papers on this subject by Dr. Beale,
in ‘Medical Times and Gazette,’ Sept. 9th, 1865, pp. 273, 330.
[47] Third Report of the R. Comm. on the Cattle Plague, as quoted in
‘Gardener’s Chronicle,’ 1866, p. 446.
[48] Mr. F. Buckland found 6,867,840 eggs in a cod-fish (‘Land and
Water,’ 1868, p. 62). An Ascaris produces about 64,000,000 eggs
(Carpenter’s ‘Comp. Phys.,’ 1854, p. 590). Mr. J. Scott, of the Royal
Botanic Garden of Edinburgh, calculated, in the same manner as I have
done for some British Orchids (‘Fertilisation of Orchids,’ p. 344),
the number of seeds in a capsule of an Acropera and found the number
to be 371,250. Now this plant produces several flowers on a raceme,
and many racemes during a season. In an allied genus, Gongora, Mr.
Scott has seen twenty capsules produced on a single raceme; ten such
racemes on the Acropera would yield above seventy-four millions of
seed.
[49] Paget, ‘Lectures on Pathology,’ p. 27; Virchow, ‘Cellular
Pathology,’ translated by Dr. Chance, pp. 123, 126, 294. Claude
Bernard, ‘Des Tissus Vivants,’ pp. 177, 210, 337; Müller,
‘Physiology,’ Eng. translat., p. 290.
[50] Prof. Ray Lankester has discussed several of the points here
referred to as bearing on pangenesis, in his interesting essay, ‘On
Comparative Longevity in Man and the Lower Animals,’ 1870, pp. 33, 77,
etc.
[51] Dr. Ross refers to this subject in his ‘Graft Theory of Disease,’
1872, p. 53.
[52] Virchow, ‘Cellular Pathology,’ translated by Dr. Chance, 1860,
pp. 60, 162, 245, 441, 454.
[53] Ibid., pp. 412-426.
[54] _See_ some good criticisms on this head by Delpino and by Mr. G.
H. Lewes in the ‘Fortnightly Review,’ Nov. 1st, 1868, p. 509.
[55] Mr. Herbert Spencer (‘Principles of Biology,’ vol. ii., p. 430)
has fully discussed this antagonism.
[56] The male salmon is known to breed at a very early age. The Triton
and Siredon, whilst retaining their larval branchiæ, according to
Filippi and Duméril (‘Annals and Mag. of Nat. Hist.,’ 3rd series,
1866, p. 157) are capable of reproduction. Ernst Haeckel has recently
(‘Monatsbericht Akad. Wiss. Berlin,’ Feb. 2nd, 1865) observed the
surprising case of a medusa, with its reproductive organs active,
which produces by budding a widely different form of medusa; and this
latter also has the power of sexual reproduction. Krohn has shown
(‘Annals and Mag. of Nat. Hist.,’ 3rd series, vol. xix., 1862, p. 6)
that certain other medusæ, whilst sexually mature, propagate by gemmæ.
_See also_ Kolliker, ‘Morphologie und Entwickelungsgeschichte des
Pennatulidenstammes,’ 1872, p. 12.
[57] _See_ his excellent discussion on this subject in ‘Nouvelles
Archives du Museum,’ tom. i., p. 151.
[58] ‘Proc. Boston Soc. of Nat. Hist.,’ republished in ‘Scientific
Opinion,’ Nov. 10th, 1869, p. 488.
[59] Todd’s ‘Cyclop. of Anat. and Phys.,’ vol. iv., 1849-52, p. 975.
[60] ‘Compte Rendus,’ Nov. 14th, 1865, p. 800.
[61] As previously remarked by Quatrefages, in his ‘Métamorphoses de
l’Homme,’ etc., 1862, p. 129.
[62] Günther’s ‘Zoological Record,’ 1864, p. 279.
[63] Sedgwick, ‘Medico-Chirurg. Review,’ April 1863, p. 454.
[64] Isid. Geoffroy Saint-Hilaire, ‘Hist. des Anomalies,’ tom. i.,
1832, pp. 435, 657; and tom. ii., p. 560.
[65] Virchow, ‘Cellular Pathology,’ 1860, p. 66.
[66] Müller’s ‘Phys.,’ Eng. Translat., vol. i., 1833, p. 407. A case
of this kind has lately been communicated to me.
[67] Dr. Fürbringer, ‘Die Knochen etc. bei den schlangenähnlichen
Sauriern,’ as reviewed in ‘Journal of Anat. and Phys.,’ May 1870, p.
286.
[68] Moquin-Tandon, ‘Tératologie Vég.,’ 1841, pp. 218, 220, 353. For
the case of the pea, _see_ ‘Gardener’s Chronicle,’ 1866, p. 897. With
respect to pollen within ovules, _see_ Dr. Masters in ‘Science
Review,’ Oct. 1873, p. 369. The Rev. J. M. Berkeley describes a bud
developed on a petal of a Clarkia, in ‘Gardener’s Chronicle,’ April
28th, 1866.
[69] _See_ some remarks to this effect by Sir H. Holland in his
‘Medical Notes,’ 1839, p. 32.
[70] This is the view taken by Prof. Haeckel, in his ‘Generelle
Morphologie’ (B. ii. s. 171), who says: “Lediglich die partielle
Identität der specifisch constituirten Materie im elterlichen und im
kindlichen Organismus, die Theilung dieser Materie bei der
Fortpflanzung, ist die Ursache der Erblichkeit.”
[71] In these remarks I, in fact, follow Naudin, who speaks of the
elements or essences of the two species which are crossed. _ See_ his
excellent memoir in the ‘Nouvelles Archives du Muséum,’ tom. i., p.
151.
CHAPTER XXVIII. CONCLUDING REMARKS.
DOMESTICATION—NATURE AND CAUSES OF VARIABILITY—SELECTION—DIVERGENCE AND
DISTINCTNESS OF CHARACTER—EXTINCTION OF RACES—CIRCUMSTANCES FAVOURABLE
TO SELECTION BY MAN—ANTIQUITY OF CERTAIN RACES—THE QUESTION WHETHER
EACH PARTICULAR VARIATION HAS BEEN SPECIALLY PREORDAINED.
As summaries have been added to nearly all the chapters, and as, in the
chapter on pangenesis, various subjects, such as the forms of
reproduction, inheritance, reversion, the causes and laws of
variability, etc., have been recently discussed, I will here only make
a few general remarks on the more important conclusions which may be
deduced from the multifarious details given throughout this work.
Savages in all parts of the world easily succeed in taming wild
animals; and those inhabiting any country or island, when first visited
by man, would probably have been still more easily tamed. Complete
subjugation generally depends on an animal being social in its habits,
and on receiving man as the chief of the herd or family. In order that
an animal should be domesticated it must be fertile under changed
conditions of life, and this is far from being always the case. An
animal would not have been worth the labour of domestication, at least
during early times, unless of service to man. From these circumstances
the number of domesticated animals has never been large. With respect
to plants, I have shown in the ninth chapter how their varied uses were
probably first discovered, and the early steps in their cultivation.
Man could not have known, when he first domesticated an animal or
plant, whether it would flourish and multiply when transported to other
countries, therefore he could not have been thus influenced in his
choice. We see that the close adaptation of the reindeer and camel to
extremely cold and hot countries has not prevented their domestication.
Still less could man have foreseen whether his animals and plants would
vary in succeeding generations and thus give birth to new races; and
the small capacity of variability in the goose has not prevented its
domestication from a remote epoch.
With extremely few exceptions, all animals and plants which have been
long domesticated have varied greatly. It matters not under what
climate, or for what purpose they are kept, whether as food for man or
beast, for draught or hunting, for clothing or mere pleasure,—under all
these circumstances races have been produced which differ more from one
another than do the forms which in a state of nature are ranked as
different species. Why certain animals and plants have varied more
under domestication than others we do not know, any more than why some
are rendered more sterile than others under changed conditions of life.
But we have to judge of the amount of variation which our domestic
productions have undergone, chiefly by the number and amount of
difference between the races which have been formed, and we can often
clearly see why many and distinct races have not been formed, namely,
because slight successive variations have not been steadily
accumulated; and such variations will never be accumulated if an animal
or plant be not closely observed, much valued, and kept in large
numbers.
The fluctuating, and, as far as we can judge, never-ending variability
of our domesticated productions,—the plasticity of almost their whole
organisation,--is one of the most important lessons which we learn from
the numerous details given in the earlier chapters of this work. Yet
domesticated animals and plants can hardly have been exposed to greater
changes in their conditions of life than have many natural species
during the incessant geological, geographical, and climatal changes to
which the world has been subject; but domesticated productions will
often have been exposed to more sudden changes and to less continuously
uniform conditions. As man has domesticated so many animals and plants
belonging to widely different classes, and as he certainly did not
choose with prophetic instinct those species which would vary most, we
may infer that all natural species, if exposed to analogous conditions,
would, on an average, vary to the same degree. Few men at the present
day will maintain that animals and plants were created with a tendency
to vary, which long remained dormant, in order that fanciers in after
ages might rear, for instance, curious breeds of the fowl, pigeon, or
canary-bird.
From several causes it is difficult to judge of the amount of
modification which our domestic productions have undergone. In some
cases the primitive parent-stock has become extinct; or it cannot be
recognised with certainty, owing to its supposed descendants having
been so much modified. In other cases two or more closely-allied forms,
after being domesticated, have crossed; and then it is difficult to
estimate how much of the character of the present descendants ought to
be attributed to variation, and how much to the influence of the
several parent-stocks. But the degree to which our domesticated breeds
have been modified by the crossing of distinct species has probably
been much exaggerated by some authors. A few individuals of one form
would seldom permanently affect another form existing in greater
numbers; for, without careful selection, the stain of the foreign blood
would soon be obliterated, and during early and barbarous times, when
our animals were first domesticated, such care would seldom have been
taken.
There is good reason to believe in the case of the dog, ox, pig, and of
some other animals, that several of our races are descended from
distinct wild prototypes; nevertheless the belief in the multiple
origin of our domesticated animals has been extended by some few
naturalists and by many breeders to an unauthorised extent. Breeders
refuse to look at the whole subject under a single point of view; I
have heard it said by a man, who maintained that our fowls were
descended from at least half-a-dozen aboriginal species, that the
evidence of the common origin of pigeons, ducks and rabbits, was of no
avail with respect to fowls. Breeders overlook the improbability of
many species having been domesticated at an early and barbarous period.
They do not consider the improbability of species having existed in a
state of nature which, if they resembled our present domestic breeds,
would have been highly abnormal in comparison with all their congeners.
They maintain that certain species, which formerly existed, have become
extinct, or are now unknown, although formerly known. The assumption of
so much recent extinction is no difficulty in their eyes; for they do
not judge of its probability by the facility or difficulty of the
extinction of other closely-allied wild forms. Lastly, they often
ignore the whole subject of geographical distribution as completely as
if it were the result of chance.
Although from the reasons just assigned it is often difficult to judge
accurately of the amount of change which our domesticated productions
have undergone, yet this can be ascertained in the cases in which all
the breeds are known to be descended from a single species,—as with the
pigeon, duck, rabbit, and almost certainly with the fowl; and by the
aid of analogy this can be judged of to a certain extent with
domesticated animals descended from several wild stocks. It is
impossible to read the details given in the earlier chapters and in
many published works, or to visit our various exhibitions, without
being deeply impressed with the extreme variability of our domesticated
animals and cultivated plants. No part of the organisation escapes the
tendency to vary. The variations generally affect parts of small vital
or physiological importance, but so it is with the differences which
exist between closely-allied species. In these unimportant characters
there is often a greater difference between the breeds of the same
species than between the natural species of the same genus, as Isidore
Geoffroy has shown to be the case with size, and as is often the case
with the colour, texture, form, etc., of the hair, feathers, horns, and
other dermal appendages.
It has often been asserted that important parts never vary under
domestication, but this is a complete error. Look at the skull of the
pig in any one of the highly improved breeds, with the occipital
condyles and other parts greatly modified; or look at that of the niata
ox. Or, again, in the several breeds of the rabbit, observe the
elongated skull, with the differently shaped occipital foramen, atlas,
and other cervical vertebrae. The whole shape of the brain, together
with the skull, has been modified in Polish fowls; in other breeds of
the fowl the number of the vertebrae and the forms of the cervical
vertebrae have been changed. In certain pigeons the shape of the lower
jaw, the relative length of the tongue, the size of the nostrils and
eyelids, the number and shape of the ribs, the form and size of the
oesophagus, have all varied. In certain quadrupeds the length of the
intestines has been much increased or diminished. With plants we see
wonderful differences in the stones of various fruits. In the
Cucurbitaceae several highly important characters have varied, such as
the sessile position of the stigmas on the ovarium, the position of the
carpels, and the projection of the ovarium out of the receptacle. But
it would be useless to run through the many facts given in the earlier
chapters.
It is notorious how greatly the mental disposition, tastes, habits,
consensual movements, loquacity or silence, and tone of voice have
varied and been inherited in our domesticated animals. The dog offers
the most striking instance of changed mental attributes, and these
differences cannot be accounted for by descent from distinct wild
types.
New characters may appear and old ones disappear at any stage of
development, being inherited at a corresponding stage. We see this in
the difference between the eggs, the down on the chickens and the first
plumage of the various breeds of the fowl; and still more plainly in
the differences between the caterpillars and cocoons of the various
breeds of the silk-moth. These facts, simple as they appear, throw
light on the differences between the larval and adult states of allied
natural species, and on the whole great subject of embryology. New
characters first appearing late in life are apt to become attached
exclusively to that sex in which they first arose, or they may be
developed in a much higher degree in this than in the other sex; or
again, after having become attached to one sex, they may be transferred
to the opposite sex. These facts, and more especially the circumstance
that new characters seem to be particularly liable, from some unknown
cause, to become attached to the male sex, have an important bearing on
the acquirement of secondary sexual characters by animals in a state of
nature.
It has sometimes been said that our domestic races do not differ in
constitutional peculiarities, but this cannot be maintained. In our
improved cattle, pigs, etc., the period of maturity, including that of
the second dentition, has been much hastened. The period of gestation
varies much, and has been modified in a fixed manner in one or two
cases. In some breeds of poultry and pigeons the period at which the
down and the first plumage are acquired, differs. The number of moults
through which the larvae of silk-moths pass, varies. The tendency to
fatten, to yield much milk, to produce many young or eggs at a birth or
during life, differs in different breeds. We find different degrees of
adaptation to climate, and different tendencies to certain diseases, to
the attacks of parasites, and to the action of certain vegetable
poisons. With plants, adaptation to certain soils, the power of
resisting frost, the period of flowering and fruiting, the duration of
life, the period of shedding the leaves or of retaining them throughout
the winter, the proportion and nature of certain chemical compounds in
the tissues or seeds, all vary.
There is, however, one important constitutional difference between
domestic races and species; I refer to the sterility which almost
invariably follows, in a greater or less degree, when species are
crossed, and to the perfect fertility of the most distinct domestic
races, with the exception of a very few plants, when similarly crossed.
It is certainly a most remarkable fact that many closely-allied
species, which in appearance differ extremely little, should yield when
crossed only a few more or less sterile offspring, or none at all;
whilst domestic races which differ conspicuously from each other are,
when united, remarkably fertile, and yield perfectly fertile offspring.
But this fact is not in reality so inexplicable as it at first appears.
In the first place, it was clearly shown in the nineteenth chapter that
the sterility of crossed species does not depend chiefly on differences
in their external structure or general constitution, but on differences
in the reproductive system, analogous to those which cause the lessened
fertility of the illegitimate unions of dimorphic and trimorphic
plants. In the second place, the Pallasian doctrine, that species after
having been long domesticated lose their natural tendency to sterility
when crossed, has been shown to be highly probable or almost certain.
We cannot avoid this conclusion when we reflect on the parentage and
present fertility of the several breeds of the dog, of the Indian or
humped and European cattle, and of the two chief kinds of pigs. Hence
it would be unreasonable to expect that races formed under
domestication should acquire sterility when crossed, whilst at the same
time we admit that domestication eliminates the normal sterility of
crossed species. Why with closely-allied species their reproductive
systems should almost invariably have been modified in so peculiar a
manner as to be mutually incapable of acting on each other—though in
unequal degrees in the two sexes, as shown by the difference in
fertility between reciprocal crosses of the same species—we do not
know, but may with much probability infer the cause to be as follows.
Most natural species have been habituated to nearly uniform conditions
of life for an incomparably longer time than have domestic races; and
we positively know that changed conditions exert an especial and
powerful influence on the reproductive system. Hence this difference
may well account for the difference in the power of reproduction
between domestic races when crossed and species when crossed. It is
probably in chief part owing to the same cause that domestic races can
be suddenly transported from one climate to another, or placed under
widely different conditions, and yet retain in most cases their
fertility unimpaired; whilst a multitude of species subjected to lesser
changes are rendered incapable of breeding.
The offspring of crossed domestic races and of crossed species resemble
each other in most respects, with the one important exception of
fertility; they often partake in the same unequal degree of the
characters of their parents, one of which is often prepotent over the
other; and they are liable to reversion of the same kind. By successive
crosses one species may be made to absorb completely another, and so it
notoriously is with races. The latter resemble species in many other
ways. They sometimes inherit their newly-acquired characters almost or
even quite as firmly as species. The conditions leading to variability
and the laws governing its nature appear to be the same in both.
Varieties can be classed in groups under groups, like species under
genera, and these under families and orders; and the classification may
be either artificial,—that is, founded on any arbitrary character,—or
natural. With varieties a natural classification is certainly founded,
and with species is apparently founded, on community of descent,
together with the amount of modification which the forms have
undergone. The characters by which domestic varieties differ from one
another are more variable than those distinguishing species, though
hardly more so than with certain polymorphic species; but this greater
degree of variability is not surprising, as varieties have generally
been exposed within recent times to fluctuating conditions of life, and
are much more liable to have been crossed; they are also in many cases
still undergoing, or have recently undergone, modification by man’s
methodical or unconscious selection.
Domestic varieties as a general rule certainly differ from one another
in less important parts than do species; and when important differences
occur, they are seldom firmly fixed; but this fact is intelligible, if
we consider man’s method of selection. In the living animal or plant he
cannot observe internal modifications in the more important organs; nor
does he regard them as long as they are compatible with health and
life. What does the breeder care about any slight change in the molar
teeth of his pigs, or for an additional molar tooth in the dog; or for
any change in the intestinal canal or other internal organ? The breeder
cares for the flesh of his cattle being well marbled with fat, and for
an accumulation of fat within the abdomen of his sheep, and this he has
effected. What would the floriculturist care for any change in the
structure of the ovarium or of the ovules? As important internal organs
are certainly liable to numerous slight variations, and as these would
probably be transmitted, for many strange monstrosities are inherited,
man could undoubtedly effect a certain amount of change in these
organs. When he has produced any modification in an important part, he
has generally done so unintentionally, in correlation with some other
conspicuous part. For instance, he has given ridges and protuberances
to the skulls of fowls, by attending to the form of the comb, or to the
plume of feathers on the head. By attending to the external form of the
pouter-pigeon, he has enormously increased the size of the oesophagus,
and has added to the number of the ribs, and given them greater
breadth. With the carrier-pigeon, by increasing through steady
selection the wattles on the upper mandible, he has greatly modified
the form of the lower mandible; and so in many other cases. Natural
species, on the other hand, have been modified exclusively for their
own good, to fit them for infinitely diversified conditions of life, to
avoid enemies of all kinds, and to struggle against a host of
competitors. Hence, under such complex conditions, it would often
happen that modifications of the most varied kinds, in important as
well as in unimportant parts, would be advantageous or even necessary;
and they would slowly but surely be acquired through the survival of
the fittest. Still more important is the fact that various indirect
modifications would likewise arise through the law of correlated
variation.
Domestic breeds often have an abnormal or semi-monstrous character, as
amongst dogs, the Italian greyhound, bulldog, Blenheim spaniel, and
bloodhound,—some breeds of cattle and pigs,—several breeds of the
fowl,—and the chief breeds of the pigeon. In such abnormal breeds,
parts which differ but slightly or not at all in the allied natural
species, have been greatly modified. This may be accounted for by man’s
often selecting, especially at first, conspicuous and semi-monstrous
deviations of structure. We should, however, be cautious in deciding
what deviations ought to be called monstrous: there can hardly be a
doubt that, if the brush of horse-like hair on the breast of the
turkey-cock had first appeared in the domesticated bird, it would have
been considered as a monstrosity; the great plume of feathers on the
head of the Polish cock has been thus designated, though plumes are
common on the heads of many kinds of birds; we might call the wattle or
corrugated skin round the base of the beak of the English
carrier-pigeon a monstrosity, but we do not thus speak of the globular
fleshy excrescence at the base of the beak of the _Carpophaga
oceanica._
Some authors have drawn a wide distinction between artificial and
natural breeds; although in extreme cases the distinction is plain, in
many other cases it is arbitrary; the difference depending chiefly on
the kind of selection which has been applied. Artificial breeds are
those which have been intentionally improved by man; they frequently
have an unnatural appearance, and are especially liable to lose their
characters through reversion and continued variability. The so-called
natural breeds, on the other hand, are those which are found in
semi-civilised countries, and which formerly inhabited separate
districts in nearly all the European kingdoms. They have been rarely
acted on by man’s intentional selection; more frequently by unconscious
selection, and partly by natural selection, for animals kept in
semi-civilised countries have to provide largely for their own wants.
Such natural breeds will also have been directly acted on by the
differences, though slight, in the surrounding conditions.
There is a much more important distinction between our several breeds,
namely, in some having originated from a strongly-marked or
semi-monstrous deviation of structure, which, however, may subsequently
have been augmented by selection; whilst others have been formed in so
slow and insensible a manner, that if we could see their early
progenitors we should hardly be able to say when or how the breed first
arose. From the history of the racehorse, greyhound, gamecock, etc.,
and from their general appearance, we may feel nearly confident that
they were formed by a slow process of improvement; and we know that
this has been the case with the carrier-pigeon, as well as with some
other pigeons. On the other hand, it is certain that the ancon and
mauchamp breeds of sheep, and almost certain that the niata cattle,
turnspit, and pug-dogs, jumper and frizzled fowls, short-faced tumbler
pigeons, hook-billed ducks, etc., suddenly appeared in nearly the same
state as we now see them. So it has been with many cultivated plants.
The frequency of these cases is likely to lead to the false belief that
natural species have often originated in the same abrupt manner. But we
have no evidence of the appearance, or at least of the continued
procreation, under nature, of abrupt modifications of structure; and
various general reasons could be assigned against such a belief.
On the other hand, we have abundant evidence of the constant occurrence
under nature of slight individual differences of the most diversified
kinds; and we are thus led to conclude that species have generally
originated by the natural selection of extremely slight differences.
This process may be strictly compared with the slow and gradual
improvement of the racehorse, greyhound, and gamecock. As every detail
of structure in each species has to be closely adapted to its habits of
life, it will rarely happen that one part alone will be modified; but,
as was formerly shown, the co-adapted modifications need not be
absolutely simultaneous. Many variations, however, are from the first
connected by the law of correlation. Hence it follows that even
closely-allied species rarely or never differ from one another by one
character alone; and the same remark is to a certain extent applicable
to domestic races; for these, if they differ much, generally differ in
many respects.
Some naturalists boldly insist[1] that species are absolutely distinct
productions, never passing by intermediate links into one another;
whilst they maintain that domestic varieties can always be connected
either with one another or with their parent-forms. But if we could
always find the links between the several breeds of the dog, horse,
cattle, sheep, pigs, etc., there would not have been such incessant
doubts whether they were descended from one or several species. The
greyhound genus, if such a term may be used, cannot be closely
connected with any other breed, unless, perhaps, we go back to the
ancient Egyptian monuments. Our English bulldog also forms a very
distinct breed. In all these cases crossed breeds must of course be
excluded, for distinct natural species can thus be likewise connected.
By what links can the Cochin fowl be closely united with others? By
searching for breeds still preserved in distant lands, and by going
back to historical records, tumbler-pigeons, carriers, and barbs can be
closely connected with the parent rock-pigeon; but we cannot thus
connect the turbit or the pouter. The degree of distinctness between
the various domestic breeds depends on the amount of modification which
they have undergone, and more especially on the neglect and final
extinction of intermediate and less-valued forms.
It has often been argued that no light is thrown on the changes which
natural species are believed to undergo from the admitted changes of
domestic races, as the latter are said to be mere temporary
productions, always reverting, as soon as they become feral, to their
pristine form. This argument has been well combated by Mr. Wallace[2]
and full details were given in the thirteenth chapter, showing that the
tendency to reversion in feral animals and plants has been greatly
exaggerated, though no doubt it exists to a certain extent. It would be
opposed to all the principles inculcated in this work, if domestic
animals, when exposed to new conditions and compelled to struggle for
their own wants against a host of foreign competitors, were not
modified in the course of time. It should also be remembered that many
characters lie latent in all organic beings, ready to be evolved under
fitting conditions; and in breeds modified within recent times, the
tendency to reversion is particularly strong. But the antiquity of some
of our breeds clearly proves that they remain nearly constant as long
as their conditions of life remain the same.
It has been boldly maintained by some authors that the amount of
variation to which our domestic productions are liable is strictly
limited; but this is an assertion resting on little evidence. Whether
or not the amount of change in any particular direction is limited, the
tendency to general variability is, as far as we can judge, unlimited.
Cattle, sheep, and pigs have varied under domestication from the
remotest period, as shown by the researches of Rutimeyer and others;
yet these animals have been improved to an unparalleled degree, within
quite recent times, and this implies continued variability of
structure. Wheat, as we know from the remains found in the Swiss
lake-dwellings, is one of the most anciently cultivated plants, yet at
the present day new and better varieties frequently arise. It may be
that an ox will never be produced of larger size and finer proportions,
or a racehorse fleeter, than our present animals, or a gooseberry
larger than the London variety; but he would be a bold man who would
assert that the extreme limit in these respects has been finally
attained. With flowers and fruit it has repeatedly been asserted that
perfection has been reached, but the standard has soon been excelled. A
breed of pigeons may never be produced with a beak shorter than that of
the present short-faced tumbler, or with one longer than that of the
English carrier, for these birds have weak constitutions and are bad
breeders; but shortness and length of beak are the points which have
been steadily improved during the last 150 years, and some of the best
judges deny that the goal has yet been reached. From reasons which
could be assigned, it is probable that parts which have now reached
their maximum development, might, after remaining constant during a
long period, vary again in the direction of increase under new
conditions of life. But there must be, as Mr. Wallace has remarked with
much truth,[3] a limit to change in certain directions both with
natural and domestic productions; for instance, there must be a limit
to the fleetness of any terrestrial animal, as this will be determined
by the friction to be overcome, the weight to be carried, and the power
of contraction in the muscular fibres. The English racehorse may have
reached this limit; but it already surpasses in fleetness its own wild
progenitor and all other equine species. The short-faced tumbler-pigeon
has a beak shorter, and the carrier a beak longer, relatively to the
size of their bodies, than that of any natural species of the family.
Our apples, pears and gooseberries bear larger fruit than those of any
natural species of the same genera; and so in many other cases.
It is not surprising, seeing the great difference between many domestic
breeds, that some few naturalists have concluded that each is descended
from a distinct aboriginal stock, more especially as the principle of
selection has been ignored, and the high antiquity of man, as a breeder
of animals, has only recently become known. Most naturalists, however,
freely admit that our various breeds, however dissimilar, are descended
from a single stock, although they do not know much about the art of
breeding, cannot show the connecting links, nor say where and when the
breeds arose. Yet these same naturalists declare, with an air of
philosophical caution, that they will never admit that one natural
species has given birth to another until they behold all the
transitional steps. Fanciers use exactly the same language with respect
to domestic breeds; thus, an author of an excellent treatise on pigeons
says he will never allow that the carrier and fantail are the
descendants of the wild rock-pigeon, until the transitions have
“actually been observed, and can be repeated whenever man chooses to
set about the task.” No doubt it is difficult to realise that slight
changes added up during long centuries can produce such great results;
but he who wishes to understand the origin of domestic breeds or of
natural species must overcome this difficulty.
The causes which excite and the laws which govern variability have been
discussed so lately, that I need here only enumerate the leading
points. As domesticated organisms are much more liable to slight
deviations of structure and to monstrosities than species living under
their natural conditions, and as widely-ranging species generally vary
more than those which inhabit restricted areas, we may infer that
variability mainly depends on changed conditions of life. We must not
overlook the effects of the unequal combination of the characters
derived from both parents, or reversion to former progenitors. Changed
conditions have an especial tendency to render the reproductive organs
more or less impotent, as shown in the chapter devoted to this subject;
and these organs consequently often fail to transmit faithfully the
parental characters. Changed conditions also act directly and
definitely on the organisation, so that all or nearly all the
individuals of the same species thus exposed become modified in the
same manner; but why this or that part is especially affected we can
seldom or ever say. In most cases, however, a change in the conditions
seems to act indefinitely, causing diversified variations in nearly the
same manner as exposure to cold or the absorption of the same poison
affects different individuals in different ways. We have reason to
suspect that an habitual excess of highly-nutritious food, or an excess
relatively to the wear and tear of the organisation from exercise, is a
powerful exciting cause of variability. When we see the symmetrical and
complex outgrowths, caused by a minute drop of the poison of a
gall-insect, we may believe that slight changes in the chemical nature
of the sap or blood would lead to extraordinary modifications of
structure.
The increased use of a muscle with its various attached parts, and the
increased activity of a gland or other organ, lead to their increased
development. Disuse has a contrary effect. With domesticated
productions, although their organs sometimes become rudimentary through
abortion, we have no reason to suppose that this has ever followed
solely from disuse. With natural species, on the contrary, many organs
appear to have been rendered rudimentary through disuse, aided by the
principle of the economy of growth together with intercrossing.
Complete abortion can be accounted for only by the hypothesis given in
the last chapter, namely, the final destruction of the germs or
gemmules of useless parts. This difference between species and domestic
varieties may be partly accounted for by disuse having acted on the
latter for an insufficient length of time, and partly from their
exemption from any severe struggle for existence entailing rigid
economy in the development of each part, to which all species under
nature are subjected. Nevertheless the law of compensation or
balancement, which likewise depends on the economy of growth,
apparently has affected to a certain extent our domesticated
productions.
As almost every part of the organisation becomes highly variable under
domestication, and as variations are easily selected both consciously
and unconsciously, it is very difficult to distinguish between the
effects of the selection of indefinite variations and the direct action
of the conditions of life. For instance, it is possible that the feet
of our water-dogs and of the American dogs which have to travel much
over the snow, may have become partially webbed from the stimulus of
widely extending their toes; but it is more probable that the webbing,
like the membrane between the toes of certain pigeons, spontaneously
appeared and was afterwards increased by the best swimmers and the best
snow-travellers being preserved during many generations. A fancier who
wished to decrease the size of his bantams or tumbler-pigeons would
never think of starving them, but would select the smallest individuals
which spontaneously appeared. Quadrupeds are sometimes born destitute
of hair and hairless breeds have been formed, but there is no reason to
believe that this is caused by a hot climate. Within the tropics heat
often causes sheep to lose their fleeces; on the other hand, wet and
cold act as a direct stimulus to the growth of hair; but who will
pretend to decide how far the thick fur of arctic animals, or their
white colour, is due to the direct action of a severe climate, and how
far to the preservation of the best-protected individuals during a long
succession of generations?
Of all the laws governing variability, that of correlation is one of
the most important. In many cases of slight deviations of structure as
well as of grave monstrosities, we cannot even conjecture what is the
nature of the bond of connexion. But between homologous parts—between
the fore and hind limbs—between the hair, hoofs, horns, and teeth—which
are closely similar during their early development and which are
exposed to similar conditions, we can see that they would be eminently
liable to be modified in the same manner. Homologous parts, from having
the same nature, are apt to blend together, and, when many exist, to
vary in number.
Although every variation is either directly or indirectly caused by
some change in the surrounding conditions, we must never forget that
the nature of the organisation which is acted on, is by far the more
important factor in the result. We see this in different organisms,
which when placed under similar conditions vary in a different manner,
whilst closely-allied organisms under dissimilar conditions often vary
in nearly the same manner. We see this, in the same modification
frequently reappearing in the same variety at long intervals of time,
and likewise in the several striking cases given of analogous or
parallel variations. Although some of these latter cases are due to
reversion, others cannot thus be accounted for.
From the indirect action of changed conditions on the organisation,
owing to the reproductive organs being thus affected—from the direct
action of such conditions, and these will cause the individuals of the
same species either to vary in the same manner, or differently in
accordance with slight differences in their constitution—from the
effects of the increased or decreased use of parts—and from
correlation,—the variability of our domesticated productions is
complicated to an extreme degree. The whole organisation becomes
slightly plastic. Although each modification must have its own exciting
cause, and though each is subjected to law, yet we can so rarely trace
the precise relation between cause and effect, that we are tempted to
speak of variations as if they arose spontaneously. We may even call
them accidental, but this must be only in the sense in which we say
that a fragment of rock dropped from a height owes its shape to
accident.
It may be worth while briefly to consider the result of the exposure to
unnatural conditions of a large number of animals of the same species
and allowed to cross freely with no selection of any kind, and
afterwards to consider the result when selection is brought into play.
Let us suppose that 500 wild rock-pigeons were confined in their native
land in an aviary and fed in the same manner as pigeons usually are;
and that they were not allowed to increase in number. As pigeons
propagate so rapidly, I suppose that a thousand or fifteen hundred
birds would have to be annually killed. After several generations had
been thus reared, we may feel sure that some of the young birds would
vary, and the variations would tend to be inherited; for at the present
day slight deviations of structure often occur and are inherited. It
would be tedious even to enumerate the multitude of points which still
go on varying or have recently varied. Many variations would occur in
correlation with one another, as the length of the wing and tail
feathers—the number of the primary wing-feathers, as well as the number
and breadth of the ribs, in correlation with the size and form of the
body—the number of the scutellae with the size of the feet—the length
of the tongue with the length of the beak—the size of the nostrils and
eyelids and the form of lower jaw in correlation with the development
of wattle—the nakedness of the young with the future colour of the
plumage—the size of the feet with that of the beak, and other such
points. Lastly, as our birds are supposed to be confined in an aviary,
they would use their wings and legs but little, and certain parts of
the skeleton, such as the sternum, scapulae and feet, would in
consequence become slightly reduced in size.
As in our assumed case many birds have to be indiscriminately killed
every year, the chances are against any new variety surviving long
enough to breed. And as the variations which arise are of an extremely
diversified nature, the chances are very great against two birds
pairing which have varied in the same manner; nevertheless, a varying
bird even when not thus paired would occasionally transmit its
character to its young; and these would not only be exposed to the same
conditions which first caused the variation in question to appear, but
would in addition inherit from their modified parent a tendency again
to vary in the same manner. So that, if the conditions decidedly tended
to induce some particular variation, all the birds might in the course
of time become similarly modified. But a far commoner result would be,
that one bird would vary in one way and another bird in another way;
one would be born with a beak a little longer, and another with a
shorter beak; one would gain some black feathers, another some white or
red feathers. And as these birds would be continually intercrossing,
the final result would be a body of individuals differing from each
other in many ways, but only slightly; yet more than did the original
rock-pigeons. But there would not be the least tendency towards the
formation of several distinct breeds.
If two separate lots of pigeons were treated in the manner just
described, one in England and the other in a tropical country, the two
lots being supplied with different kinds of food, would they after many
generations differ? When we reflect on the cases given in the
twenty-third chapter, and on such facts as the difference in former
times between the breeds of cattle, sheep, etc., in almost every
district of Europe, we are strongly inclined to admit that the two lots
would be differently modified through the influence of climate and
food. But the evidence on the definite action of changed conditions is
in most cases insufficient; and, with respect to pigeons, I have had
the opportunity of examining a large collection of domesticated kinds,
sent to me by Sir W. Elliot from India, and they varied in a remarkably
similar manner with our European birds.
If two distinct breeds were mingled together in equal numbers, there is
reason to suspect that they would to a certain extent prefer pairing
with their own kind; but they would often intercross. From the greater
vigour and fertility of the crossed offspring, the whole body would by
this means become interblended sooner than would otherwise have
occurred. From certain breeds being prepotent over others, it does not
follow that the interblended progeny would be strictly intermediate in
character. I have, also, proved that the act of crossing in itself
gives a strong tendency to reversion, so that the crossed offspring
would tend to revert to the state of the aboriginal rock-pigeon; and in
the course of time they would probably be not much more heterogeneous
in character than in our first case, when birds of the same breed were
confined together.
I have just said that the crossed offspring would gain in vigour and
fertility. From the facts given in the seventeenth chapter there can be
no doubt of this fact; and there can be little doubt, though the
evidence on this head is not so easily acquired, that long-continued
close interbreeding leads to evil results. With hermaphrodites of all
kinds, if the sexual elements of the same individual habitually acted
on each other, the closest possible interbreeding would be perpetual.
But we should bear in mind that the structure of all hermaphrodite
animals, as far as I can learn, permits and frequently necessitates a
cross with a distinct individual. With hermaphrodite plants we
incessantly meet with elaborate and perfect contrivances for this same
end. It is no exaggeration to assert that, if the use of the talons and
tusks of a carnivorous animal, or of the plumes and hooks on a seed,
may be safely inferred from their structure, we may with equal safety
infer that many flowers are constructed for the express purpose of
ensuring a cross with a distinct plant. From these various
considerations, not to mention the result of a long series of
experiments which I have tried, the conclusion arrived at in the
chapter just referred to—namely, that great good of some kind is
derived from the sexual concourse of distinct individuals—must be
admitted.
To return to our illustration: we have hitherto assumed that the birds
were kept down to the same number by indiscriminate slaughter; but if
the least choice be permitted in their preservation, the whole result
will be changed. Should the owner observe any slight variation in one
of his birds, and wish to obtain a breed thus characterised, he would
succeed in a surprisingly short time by careful selection. As any part
which has once varied generally goes on varying in the same direction,
it is easy, by continually preserving the most strongly marked
individuals, to increase the amount of difference up to a high,
predetermined standard of excellence. This is methodical selection.
If the owner of the aviary, without any thought of making a new breed,
simply admired, for instance, short-beaked more than long-beaked birds,
he would, when he had to reduce the number, generally kill the latter;
and there can be no doubt that he would thus in the course of time
sensibly modify his stock. It is improbable, if two men were to keep
pigeons and act in this manner, that they would prefer exactly the same
characters; they would, as we know, often prefer directly opposite
characters, and the two lots would ultimately come to differ. This has
actually occurred with strains or families of cattle, sheep, and
pigeons, which have been long kept and carefully attended to by
different breeders, without any wish on their part to form new and
distinct sub-breeds. This unconscious kind of selection will more
especially come into action with animals which are highly serviceable
to man; for every one tries to get the best dogs, horses, cows, or
sheep, without thinking about their future progeny, yet these animals
would transmit more or less surely their good qualities to their
offspring. Nor is any one so careless as to breed from his worst
animals. Even savages, when compelled from extreme want to kill some of
their animals, would destroy the worst and preserve the best. With
animals kept for use and not for mere amusement, different fashions
prevail in different districts, leading to the preservation, and
consequently to the transmission, of all sorts of trifling
peculiarities of character. The same process will have been pursued
with our fruit-trees and vegetables, for the best will always have been
the most largely cultivated, and will occasionally have yielded
seedlings better than their parents.
The different strains, just alluded to, which have been actually
produced by breeders without any wish on their part to obtain such a
result, afford excellent evidence of the power of unconscious
selection. This form of selection has probably led to far more
important results than methodical selection, and is likewise more
important under a theoretical point of view from closely resembling
natural selection. For during this process the best or most valued
individuals are not separated and prevented from crossing with others
of the same breed, but are simply preferred and preserved; yet this
inevitably leads to their gradual modification and improvement; so that
finally they prevail, to the exclusion of the old parent-form.
With our domesticated animals natural selection checks the production
of races with any injurious deviation of structure. In the case of
animals which, from being kept by savages or semi-civilised people,
have to provide largely for their own wants under different
circumstances, natural selection will have played a more important
part. Hence it probably is that they often closely resemble natural
species.
As there is no limit to man’s desire to possess animals and plants more
and more useful in any respect, and as the fancier always wishes, owing
to fashions running into extremes, to produce each character more and
more strongly pronounced, there is, through the prolonged action of
methodical and unconscious selection, a constant tendency in every
breed to become more and more different from its parent-stock; and when
several breeds have been produced and are valued for different
qualities, to differ more and more from each other. This leads to
Divergence of Character. As improved sub-varieties and races are slowly
formed, the older and less improved breeds are neglected and decrease
in number. When few individuals of any breed exist within the same
locality, close interbreeding, by lessening their vigour and fertility,
aids in their final extinction. Thus the intermediate links are lost,
and the remaining breeds gain in Distinctness of Character.
In the chapters on the Pigeon, it was proved by historical evidence and
by the existence of connecting sub-varieties in distant lands that
several breeds have steadily diverged in character, and that many old
and intermediate sub-breeds have been lost. Other cases could be
adduced of the extinction of domestic breeds, as of the Irish wolf-dog,
the old English hound, and of two breeds in France, one of which was
formerly highly valued.[4] Mr. Pickering remarks[5] that “the sheep
figured on the most ancient Egyptian monuments is unknown at the
present day; and at least one variety of the bullock, formerly known in
Egypt, has in like manner become extinct.” So it has been with some
animals and with several plants cultivated by the ancient inhabitants
of Europe during the neolithic period. In Peru, Von Tschudi[6] found in
certain tombs, apparently prior to the dynasty of the Incas, two kinds
of maize not now known in the country. With our flowers and culinary
vegetables, the production of new varieties and their extinction has
incessantly recurred. At the present time improved breeds sometimes
displace older breeds at an extraordinarily rapid rate; as has recently
occurred throughout England with pigs. The Longhorn cattle in their
native home were “suddenly swept away as if by some murderous
pestilence,” by the introduction of Shorthorns.[7]
What grand results have followed from the long-continued action of
methodical and unconscious selection, regulated to a certain extent by
natural selection, we see on every side of us. Compare the many animals
and plants which are displayed at our exhibitions with their
parent-forms when these are known, or consult old historical records
with respect to their former state. Most of our domesticated animals
have given rise to numerous and distinct races, but those which cannot
be easily subjected to selection must be excepted—such as cats, the
cochineal insect, and the hive-bee. In accordance with what we know of
the process of selection, the formation of our many races has been slow
and gradual. The man who first observed and preserved a pigeon with its
oesophagus a little enlarged, its beak a little longer, or its tail a
little more expanded than usual, never dreamed that he had made the
first step in the creation of a pouter, carrier, and fantail-pigeon.
Man can create not only anomalous breeds, but others having their whole
structure admirably co-ordinated for certain purposes, such as the
racehorse and dray-horse, or the greyhound and bulldog. It is by no
means necessary that each small change of structure throughout the
body, leading towards excellence, should simultaneously arise and be
selected. Although man seldom attends to differences in organs which
are important under a physiological point of view, yet he has so
profoundly modified some breeds, that assuredly, if found wild, they
would be ranked as distinct genera.
The best proof of what selection has effected is perhaps afforded by
the fact that whatever part or quality in any animal, and more
especially in any plant, is most valued by man, that part or quality
differs most in the several races. This result is well seen by
comparing the amount of difference between the fruits produced by the
several varieties of fruit-trees, between the flowers of our
flower-garden plants, between the seeds, roots, or leaves of our
culinary and agricultural plants, in comparison with the other and not
valued parts of the same varieties. Striking evidence of a different
kind is afforded by the fact ascertained by Oswald Heer[8] namely, that
the seeds of a large number of plants,—wheat, barley, oats, peas,
beans, lentils, poppies,—cultivated for their seed by the ancient
Lake-inhabitants of Switzerland, were all smaller than the seeds of our
existing varieties. Rütimeyer has shown that the sheep and cattle which
were kept by the earlier Lake-inhabitants were likewise smaller than
our present breeds. In the middens of Denmark, the earliest dog of
which the remains have been found was the weakest; this was succeeded
during the Bronze age by a stronger kind, and this again during the
Iron age by one still stronger. The sheep of Denmark during the Bronze
period had extraordinarily slender limbs, and the horse was smaller
than our present animal.[9] No doubt in most of these cases the new and
larger breeds were introduced from foreign lands by the immigration of
new hordes of men. But it is not probable that each larger breed, which
in the course of time has supplanted a previous and smaller breed, was
the descendant of a distinct and larger species; it is far more
probable that the domestic races of our various animals were gradually
improved in different parts of the great Europaeo-Asiatic continent,
and thence spread to other countries. This fact of the gradual increase
in size of our domestic animals is all the more striking as certain
wild or half-wild animals, such as red-deer, aurochs, park-cattle, and
boars[10] have within nearly the same period decreased in size.
The conditions favourable to selection by man are,—the closest
attention to every character,—long-continued perseverance,—facility in
matching or separating animals,—and especially a large number being
kept, so that the inferior individuals may be freely rejected or
destroyed, and the better ones preserved. When many are kept there will
also be a greater chance of the occurrence of well-marked deviations of
structure. Length of time is all-important; for as each character, in
order to become strongly pronounced, has to be augmented by the
selection of successive variations of the same kind, this can be
effected only during a long series of generations. Length of time will,
also, allow any new feature to become fixed by the continued rejection
of those individuals which revert or vary, and by the preservation of
those which still inherit the new character. Hence, although some few
animals have varied rapidly in certain respects under new conditions of
life, as dogs in India and sheep in the West Indies, yet all the
animals and plants which have produced strongly marked races were
domesticated at an extremely remote epoch, often before the dawn of
history. As a consequence of this, no record has been preserved of the
origin of our chief domestic breeds. Even at the present day new
strains or sub-breeds are formed so slowly that their first appearance
passes unnoticed. A man attends to some particular character, or merely
matches his animals with unusual care, and after a time a slight
difference is perceived by his neighbours;—the difference goes on being
augmented by unconscious and methodical selection, until at last a new
sub-breed is formed, receives a local name, and spreads; but by this
time its history is almost forgotten. When the new breed has spread
widely, it gives rise to new strains and sub-breeds, and the best of
these succeed and spread, supplanting other and older breeds; and so
always onwards in the march of improvement.
When a well-marked breed has once been established, if not supplanted
by still further improved sub-breeds, and if not exposed to greatly
changed conditions of life inducing further variability or reversion to
long-lost characters, it may apparently last for an enormous period. We
may infer that this is the case from the high antiquity of certain
races; but some caution is necessary on this head, for the same
variation may appear independently after long intervals of time, or in
distant places. We may safely assume that this has occurred with the
turnspit-dog, of which one is figured on the ancient Egyptian
monuments—with the solid-hoofed swine[11] mentioned by Aristotle—with
five-toed fowls described by Columella—and certainly with the
nectarine. The dogs represented on the Egyptian monuments, about 2000
B.C., show us that some of the chief breeds then existed, but it is
extremely doubtful whether any are identically the same with our
present breeds. A great mastiff sculptured on an Assyrian tomb, 640
B.C., is said to be the same with the dog still imported from Thibet
into the same region. The true greyhound existed during the Roman
classical period. Coming down to a later period, we have seen that,
though most of the chief breeds of the pigeon existed between two and
three centuries ago, they have not all retained exactly the same
character to the present day; but this has occurred in certain cases in
which no improvement was desired, for instance, in the case of the Spot
and Indian ground-tumbler.
De Candolle[12] has fully discussed the antiquity of various races of
plants; he states that the black seeded poppy was known in the time of
Homer, the white-seeded sesamum by the ancient Egyptians, and almonds
with sweet and bitter kernels by the Hebrews; but it does not seem
improbable that some of these varieties may have been lost and
reappeared. One variety of barley and apparently one of wheat, both of
which were cultivated at an immensely remote period by the
Lake-inhabitants of Switzerland, still exist. It is said[13] that
“specimens of a small variety of gourd which is still common in the
market of Lima were exhumed from an ancient cemetery in Peru.” De
Candolle remarks that, in the books and drawings of the sixteenth
century, the principal races of the cabbage, turnip, and gourd can be
recognised: this might have been expected at so late a period, but
whether any of these plants are absolutely identical with our present
sub-varieties is not certain. It is, however, said that the Brussels
sprout, a variety which in some places is liable to degeneration, has
remained genuine for more than four centuries in the district where it
is believed to have originated.[14]
In accordance with the views maintained by me in this work and
elsewhere, not only the various domestic races, but the most distinct
genera and orders within the same great class—for instance, mammals,
birds, reptiles, and fishes—are all the descendants of one common
progenitor, and we must admit that the whole vast amount of difference
between these forms has primarily arisen from simple variability. To
consider the subject under this point of view is enough to strike one
dumb with amazement. But our amazement ought to be lessened when we
reflect that beings almost infinite in number, during an almost
infinite lapse of time, have often had their whole organisation
rendered in some degree plastic, and that each slight modification of
structure which was in any way beneficial under excessively complex
conditions of life has been preserved, whilst each which was in any way
injurious has been rigorously destroyed. And the long-continued
accumulation of beneficial variations will infallibly have led to
structures as diversified, as beautifully adapted for various purposes
and as excellently co-ordinated, as we see in the animals and plants
around us. Hence I have spoken of selection as the paramount power,
whether applied by man to the formation of domestic breeds, or by
nature to the production of species. I may recur to the metaphor given
in a former chapter: if an architect were to rear a noble and
commodious edifice, without the use of cut stone, by selecting from the
fragments at the base of a precipice wedge-formed stones for his
arches, elongated stones for his lintels, and flat stones for his roof,
we should admire his skill and regard him as the paramount power. Now,
the fragments of stone, though indispensable to the architect, bear to
the edifice built by him the same relation which the fluctuating
variations of organic beings bear to the varied and admirable
structures ultimately acquired by their modified descendants.
Some authors have declared that natural selection explains nothing,
unless the precise cause of each slight individual difference be made
clear. If it were explained to a savage utterly ignorant of the art of
building, how the edifice had been raised stone upon stone, and why
wedge-formed fragments were used for the arches, flat stones for the
roof, etc.; and if the use of each part and of the whole building were
pointed out, it would be unreasonable if he declared that nothing had
been made clear to him, because the precise cause of the shape of each
fragment could not be told. But this is a nearly parallel case with the
objection that selection explains nothing, because we know not the
cause of each individual difference in the structure of each being.
The shape of the fragments of stone at the base of our precipice may be
called accidental, but this is not strictly correct; for the shape of
each depends on a long sequence of events, all obeying natural laws; on
the nature of the rock, on the lines of deposition or cleavage, on the
form of the mountain, which depends on its upheaval and subsequent
denudation, and lastly on the storm or earthquake which throws down the
fragments. But in regard to the use to which the fragments may be put,
their shape may be strictly said to be accidental. And here we are led
to face a great difficulty, in alluding to which I am aware that I am
travelling beyond my proper province. An omniscient Creator must have
foreseen every consequence which results from the laws imposed by Him.
But can it be reasonably maintained that the Creator intentionally
ordered, if we use the words in any ordinary sense, that certain
fragments of rock should assume certain shapes so that the builder
might erect his edifice? If the various laws which have determined the
shape of each fragment were not predetermined for the builder’s sake,
can it be maintained with any greater probability that He specially
ordained for the sake of the breeder each of the innumerable variations
in our domestic animals and plants;—many of these variations being of
no service to man, and not beneficial, far more often injurious, to the
creatures themselves? Did He ordain that the crop and tail-feathers of
the pigeon should vary in order that the fancier might make his
grotesque pouter and fantail breeds? Did He cause the frame and mental
qualities of the dog to vary in order that a breed might be formed of
indomitable ferocity, with jaws fitted to pin down the bull for man’s
brutal sport? But if we give up the principle in one case,—if we do not
admit that the variations of the primeval dog were intentionally guided
in order that the greyhound, for instance, that perfect image of
symmetry and vigour, might be formed,—no shadow of reason can be
assigned for the belief that variations, alike in nature and the result
of the same general laws, which have been the groundwork through
natural selection of the formation of the most perfectly adapted
animals in the world, man included, were intentionally and specially
guided. However much we may wish it, we can hardly follow Professor Asa
Gray in his belief “that variation has been led along certain
beneficial lines,” like a stream “along definite and useful lines of
irrigation.” If we assume that each particular variation was from the
beginning of all time preordained, then that plasticity of
organisation, which leads to many injurious deviations of structure, as
well as the redundant power of reproduction which inevitably leads to a
struggle for existence, and, as a consequence, to the natural selection
or survival of the fittest, must appear to us superfluous laws of
nature. On the other hand, an omnipotent and omniscient Creator ordains
everything and foresees everything. Thus we are brought face to face
with a difficulty as insoluble as is that of free will and
predestination.
REFERENCES
[1] Godron, ‘De l’Espèce,’ 1859, tom. ii. p. 44, etc.
[2] ‘Journal Proc. Linn. Soc.,’ 1858, vol. iii. p. 60.
[3] ‘The Quarterly Journal of Science,’ Oct. 1867, p. 486.
[4] M. Rufz de Lavison, in ‘Bull. Soc. Imp. d’Acclimat.,’ Dec. 1862,
p. 1009.
[5] ‘Races of Man,’ 1850, p. 315.
[6] ‘Travels in Peru,’ Eng. translat., p. 177.
[7] Youatt on Cattle, 1834, p. 200. On Pigs, _see_ ‘Gardener’s
Chronicle,’ 1854, p. 410.
[8] ‘Die Pflanzen der Pfahlbauten,’ 1865.
[9] Morlot, ‘Soc. Vaud. des Scien. Nat.,’ Mars, 1860, p. 298.
[10] Rütimeyer, ‘Die Fauna der Pfahlbauten,’ 1861, s. 30.
[11] Godron,’De l’Espèce,’ tom. i., 1859, p. 368.
[12] ‘Geographie Botan.,’ 1855, p. 989.
[13] Pickering, ‘Races of Man,’ 1850, p. 318.
[14] ‘Journal of a Horticultural Tour,’ by a Deputation of the
Caledonian Hist. Soc., 1823, p. 293.
INDEX
ABBAS PACHA, a fancier of fantailed pigeons, 6.
ABBEY, Mr., on grafting, 18 (2);
—on mignonette, 21.
ABBOTT, Mr. Keith, on the Persian tumbler pigeon, 5.
ABBREVIATION of the facial bones, 3.
ABORTION of organs, 24, 27.
ABSORPTION of minority in crossed races, 15, 19.
ABUTILON, graft hybridisation of, 11.
ACCLIMATISATION, 24;
—of maize, 9.
ACERBI, on the fertility of domestic animals in Lapland, 16.
_Achatinella,_ 13.
_Achillea millefolium,_ bud variation in, 11.
_Aconitum napellus,_ roots of, innocuous in cold climates, 23.
_Acorus calamus,_ sterility of, 18.
ACOSTA, on fowls in South America at its discovery, 7.
_Acropera,_ number of seeds in, 27.
ADAM, M., origin of _Cytisus adami,_ 11.
ADAM, W., on consanguineous marriages, 17.
ADAMS, on hereditary diseases, 12.
ADVANCEMENT in scale of organisation, Introduction.
_Ægilops triticoides,_ observations of Fabre and Godron on, 9;
—increasing fertility of hybrids of, with wheat, 16.
_Æsculus pavia,_ tendency of, to become double, 18.
_Æthusa cynapium,_ 25.
AFFINITY, sexual elective, 19.
AFRICA, white bull from, 3;
—feral cattle in, 3;
—food-plants of savages of, 9;
—South, diversity of breeds of cattle in, 3;
—West, change in fleece of sheep in, 3.
_Agave vivipara,_ seeding of, in poor soil, 18.
AGE, changes in trees, dependent on, 11.
——, as bearing on pangenesis, 27.
AGOUTI, fertility of, in captivity, 18.
AGRICULTURE, antiquity of, 21.
_Agrostis,_ seeds of, used as food, 9.
AGUARA, 1.
AINSWORTH, Mr., on the change in the hair of animals at Angora, 24.
AKBAR KHAN, his fondness for pigeons, 6; 20.
_Alauda arvensis,_ 18.
ALBIN, on “Golden Hamburgh” fowls, 7;
—figure of the hook-billed duck, 8.
ALBINISM, 4, 12.
ALBINO, negro, attacked by insects, 21.
ALBINOES, heredity of, 12.
ALBINUS, thickness of the epidermis on the palms of the hands in man,
24.
ALCO, 1, 15.
ALDROVANDI, on rabbits, 4;
—description of the nun pigeon, 5;
—on the fondness of the Dutch for pigeons in the seventeenth
century, 6;
—notice of several varieties of pigeons, 6;
—on the breeds of fowls, 7;
—on the origin of the domestic duck, 8.
ALEFIELD, Dr., on the varieties of peas and their specific unity, 9;
—on the varieties of beans, 9.
ALEXANDER the Great, his selection of Indian cattle, 20.
ALGÆ, retrogressive metamorphosis in, 27;
—division of zoospores of, 27.
ALLEN, J., birds in United States, 23.
ALLEN, W., on feral fowls, 7, 13.
ALLMAN, Professor, on a monstrous _Saxifraga geum,_ 18;
—on the Hydroida, 27 (2).
ALMOND, 10;
—antiquity of, 28;
—bitter, not eaten by mice, 21.
_Alnus glutinosa,_ and _incana,_ hybrids of, 17.
ALPACA, selection of, 20.
_Althæa rosea,_ 11, 16.
_Amaryllis,_ 17.
_Amaryllis vittata,_ effect of foreign pollen on, 5.
AMAUROSIS, hereditary, 12.
_Amblystoma lurida,_ 27.
AMERICA, limits within which no useful plants have been furnished by,
9;
—colours of feral horses in, 2;
—North, native cultivated plants of, 9;
—skin of feral pig from, 3;
—South, variations in cattle of, 3.
AMMON, on the persistency of colour in horses, 12.
_Amygdalus persica,_ 10, 11.
_Anagallis arvensis,_ 19.
ANALOGOUS variation, 5, 22;
—in horses, 5;
—in the horse and ass, 2;
—in fowls, 7.
_Anas boschas,_ 8, 13;
—skull of, figured, 8.
“ANCON” sheep of Massachusetts, 3, 15.
ANDALUSIAN fowls, 7.
ANDALUSIAN rabbits, 4.
ANDERSON, J., on the origin of British sheep, 4;
—on the selection of qualities in cattle, 20;
—on a one-eared breed of rabbits, 4;
—on the inheritance of characters from a one-eared rabbit, and
three-legged bitch, 12;
—on the persistency of varieties of peas, 9;
—on the production of early peas by selection, 20;
—on the varieties of the potato, 9;
—on crossing varieties of the melon, 11;
—on reversion in the barberry, 11.
ANDERSON, Mr., on the reproduction of the weeping ash by seed, 12.
—on the cultivation of the tree pæony in China, 20.
ANDERSSON, Mr., on the Damara, Bechuana, and Namaqua cattle, 3;
—on the cows of the Damaras, 24;
—selection practised by the Damaras and Namaquas, 20;
—on the use of grass-seeds and the roots of reeds as food in South
Africa, 9.
_Anemone coronaria,_ doubled by selection, 20.
ANGINA pectoris, hereditary, occurring at a certain age, 14.
ANGLESEA, cattle of, 3.
ANGOLA sheep, 3.
ANGORA, change in hair of animals at, 23;
—cats of, 1 (2);
—rabbits of, 4 (2).
ANIMALS, domestication of, facilitated by fearlessness of man, 1;
—refusal of wild, to breed in captivity, 18;
—compound, individual peculiarities of, reproduced by budding, 11;
—variation by selection in useful qualities of, 20.
ANNUAL plants, rarity of bud-variation in, 11.
ANOMALIES in the osteology of the horse, 2.
ANOMALOUS breeds of pigs, 3;
—of cattle, 3.
_Anser albifrons,_ characters of, reproduced in domestic geese, 8.
_Anser ægyptiacus,_ 8, 14.
_Anser canadensis,_ 18. _Anser ferus,_ the original of the domestic
goose, 8;
—fertility of cross of, with domestic goose, 8.
ANSON, on feral fowls in the Ladrones, 7.
ANTAGONISM between growth and reproduction, 27.
_Anthemis nobilis,_ bud-variation in flowers of, 11;
—becomes single in poor soil, 18.
ANTHERS, contabescence of, 18.
ANTIGUA, cats of, 5;
—changed fleece of sheep in, 3.
_Antirrhinum majus,_ peloric, 10, 13 (2), 18;
—double-flowered, 18;
—bud-variation in, 11.
ANTS, individual recognition of, 22.
APHIDES, attacking pear-trees, 21;
—development of, 27.
APOPLEXY, hereditary, occurring at a certain age, 14.
APPLE, 10;
—fruit of, in Swiss lake-dwellings, 9;
—rendered fastigiate by heat in India, 10;
—bud-variation in the, 11;
—with dimidiate fruit, 11 (2);
—with two kinds of fruit on the same branch, 11;
—artificial fecundation of, 11;
—St. Valéry, 11, 18;
—reversion in seedlings of, 13;
—crossing of varieties of, 17;
—growth of the, in Ceylon, 21;
—winter majetin, not attacked by _ coccus,_ 21;
—flower-buds of, attacked by bullfinches, 21;
—American, change of, when grown in England, 23.
APRICOT, 10 (2);
—glands on the leaves of, 21;
—analogous variation in the, 26.
_Aquila fusca,_ copulating in captivity, 18.
_Aquilegia vulgaris,_ 10, 25.
ARAB boarhound, described by Harcourt, 1.
_Arabis blepharophylla_ and _A. soyeri,_ effects of crossing, 11.
_Aralia trifoliata,_ bud-variation in leaves of, 11.
ARAUCARIAS, young, variable resistance of, to frost, 24.
ARCHANGEL pigeon, 21.
ARCTIC regions, variability of plants and shells of, 22.
_Aria vestita,_ grafted on thorns, 11.
ARISTOPHANES, fowls mentioned by, 7.
ARISTOTLE, on solid-hoofed pigs, 3;
—domestic duck unknown to, 8;
—on the assumption of male characters by old hens, 13.
ARNI, domestication of the, 3.
ARNOLD, Mr., experiments of pollen on the maize, 11.
ARRESTS of development, 24.
ARTERIES, increase of anastomosing branches of, when tied, 24.
ARU Islands, wild pig of, 3.
ARUM, Polynesian varieties of, 22.
_Ascaris,_ number of eggs of, 27.
ASH, varieties of the, 10;
—weeping, 10;
—simple-leaved, 10;
—bud-variation in, 11;
—effects of graft upon the stock in the, 11;
—production of the blotched Breadalbane, 11;
—weeping, capricious reproduction of, by seed, 12.
_Asinus burchellii,_ 2.
_Asinus hemionus,_ 13.
_Asinus indicus,_ 13 (2).
_Asinus quagga,_ 2.
_Asinus tæniopus,_ the original of the domestic ass, 2.
ASPARAGUS, increased fertility of cultivated, 16.
ASS, early domestication of the, 2;
—breeds of, 2;
—small size of, in India, 2;
—stripes of, 2 (2);
—dislike of, to cross water, 6;
—reversion in, 13 (3);
—hybrid of the, with mare and zebra, 13;
—prepotency of the, over the horse, 14;
—crossed with wild ass, 20;
—variation and selection of the, 21.
ASSYRIAN sculpture of a mastiff, 1.
ASTERS, 12, 24.
ASTHMA, hereditary, 12, 14.
ATAVISM. _See_ Reversion.
ATHELSTAN, his care of horses, 20.
ATKINSON, Mr., on the sterility of the Tarroo silk-moth in confinement,
18.
AUBERGINE, 15.
AUDUBON, on feral hybrid ducks, 6, 13;
—on the domestication of wild ducks on the Mississippi, 8;
—on the wild cock turkey visiting domestic hens, 8;
—fertility of _Fringilla ciris_ in captivity, 18;
—fertility of _Columba migratoria_ and _leucocephala_ in captivity,
18;
—breeding of _Anser canadensis_ in captivity, 18.
AUDUBON and Bachman, on the change of coat in _Ovis montana,_ 3;
—sterility of _Sciurus cinerea_ in confinement, 18.
AURICULA, effect of seasonal conditions on the, 23;
—blooming of, 26.
AUSTRALIA, no generally useful plants derived from, 9;
—useful plants of, enumerated by Hooker, 9.
AUSTRIA, heredity of character in emperors of, 14.
AUTENRIETH, on persistency of colour in horses, 12.
AVA, horses of, 2.
_Avena fatua,_ cultivability of, 9.
‘AYEEN Akbery,’ pigeons mentioned in the, 5 (2), 6 (4).
AYRES, W. P., on bud-variation in pelargoniums, 11.
_Azalea indica,_ bud-variation in, 11.
AZARA, on the feral dogs of La Plata, 1;
—on the crossing of domestic with wild cats in Paraguay, 1;
—on hornlike processes in horses, 2;
—on curled hair in horses, 2;
—on the colours of feral horses, 2;
—on the cattle of Paraguay and La Plata, 3 (3), 22;
—on a hornless bull, 20;
—on the increase of cattle in South America, 17;
—on the growth of horns in the hornless cattle of Corrientes, 13;
—on the “Niata” cattle, 3;
—on naked quadrupeds, 23;
—on a race of black-skinned fowls in South America, 7, 20;
—on a variety of maize, 9.
BABINGTON, C. C., on the origin of the plum, 10;
—British species of the genus _ Rosa,_ 10;
—distinctness of _Viola lutea_ and _tricolor,_ 10.
BACHMANN, Mr., on the turkey, 22.
_See also_ Audubon. BADGER, breeding in confinement, 18.
“BAGADOTTEN-TAUBE,” 5.
BAILY, Mr., on the effect of selection on fowls, 20;
—on Dorking fowls, 21.
BAIRD, S., on the origin of the turkey, 8.
BAKER, Mr., on heredity in the horse, 12;
—on the degeneration of the horse by neglect, 21;
—orders of Henrys VII. and VIII. for the destruction of undersized
mares, 20.
BAKEWELL, change in the sheep effected by, 20.
BALANCEMENT, 26 (2);
—of growth, law of, 26.
BALDHEAD (pigeon), 5.
BALDNESS, in man, inherited, 25;
—with deficiency in teeth, 25 (2).
BALLANCE, Mr., on the effects of interbreeding on fowls, 17;
—on variation in the eggs of fowls, 7.
_Ballota nigra,_ transmission of variegated leaves in, 11.
BAMBOO, varieties of the, 22.
BANANA, variation of the, 10, 22;
—bud-variation in the, 11;
—sterility of the, 22.
BANTAM fowls, 7;
—Sebright, origin of, 15;
—sterility of, 16.
BARB (pigeon), 5 (2), 6, 21;
—figure of, 5;