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. (27/2. Quoted by Paget 'Lectures on Pathology' 1853 page 159.) 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 Muller 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. (27/3. Dr. Lachmann also observes ('Annals and Mag. of Nat.
History' 2nd series volume 19 1857 page 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 volume 11 page 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 pages 62, 94.) 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 (27/4. See Bonnet 'Oeuvres d'Hist. Nat.' tome 5 1781
page 339 for remarks on the budding-out of the amputated limbs of
Salamanders.); 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
gemmae. (27/5. Paget 'Lectures on Pathology' 1853 page 158.)
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." (27/6. Ibid pages 152, 164.) 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
algae, 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 (27/7.
Translated in 'Annals and Mag. of Nat. Hist.' April 1870 page 272.), 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. (27/8. Bischoff as quoted by von Siebold "Ueber Parthenogenesis"
'Sitzung der math. phys. Classe.' Munich November 4, 1871 page 240. See also
Quatrefages 'Annales des Sc. Nat. Zoolog.' 3rd series 1850 page 138.) 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 larvae of Cecidomyia are said by Leuckart (27/9. 'On the
Asexual Reproduction of Cecidomyide Larvae' translated in 'Annals and Mag. of
Nat. Hist.' March 1866 pages 167, 171.) 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. (27/10. Prof. Allman speaks ('Transact. R.
Soc. of Edinburgh' volume 26 1870 page 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.") 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 algae, according to Dr. L. Radlkofer (27/11. 'Annals and Mag. of
Nat. Hist.' 2nd series volume 20 1857 pages 153-455), 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 (27/12. 'Annales des Sc. Nat.'
3rd series 1850 tome 13.), 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 (27/13. 'Transact. Phil. Soc.' 1851
pages 196, 208, 210; 1853 pages 245, 247.), 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 Kolreuter and Gartner. This last careful
observer, after making successive trials on a Malva with more and more pollen-
grains, found (27/14. 'Beitrage zur Kenntniss' etc. 1844 s. 345.), 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 (27/15.
'Nouvelles Archives du Museum' tome 1 page 27.) 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 (27/16. As
quoted by Sir J. Lubbock in 'Nat. Hist. Review' 1862 page 345. Weijenbergh
also raised ('Nature' December 21, 1871 page 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.) 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
(27/17. 'Entwickelungsgeschichte der Siphonophora' 1869 page 73.) found that
by cutting the segmented and fertilised ova or larva of Siphonophorae (jelly-
fishes) into pieces, the smaller the pieces were, the slower was the rate of
development, and the larvae 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 regrowth and
development are all parts of one and the same great law.
REGROWTH 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 (27/18. Spallanzani 'An Essay on Animal Reproduction'
translated by Dr. Maty 1769 page 79. Bonnet 'Oeuvres d'Hist. Nat.' tome 5 part
1 4to. edition 1781 pages 343, 350.) 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. (27/19. Vulpian as quoted by Prof. Faivre 'La
Variabilite des Especes' 1868 page 112.) 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. (27/20. Dr. P. Hoy 'The American Naturalist' September
1871 page 579.)
The power of regrowth is generally much greater during the youth of an animal
or during the earlier stages of its development than during maturity. The
larvae or tadpoles of the Batrachians are capable of reproducing lost members,
but not so the adults. (27/21. Dr. Gunther in Owen 'Anatomy of Vertebrates'
volume 1 1866 page 567. Spallanzani has made similar observations.) Mature
insects have no power of regrowth, excepting in one order, whilst the larvae
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 regrowth. With the higher vertebrata, such as
birds and mammals, the power is extremely limited. (27/22. 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 volume 2 page 890) of the
regrowth of limbs in the womb in the case of man.)
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 regrowth must
be diffused throughout the whole body. Nevertheless there seems to be much
truth in the view maintained by Prof. Lessona (27/23. 'Atti della Soc. Ital.
di Sc. Nat.' volume 11 1869 page 493.), 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 regrowth, as we have just seen; and
this animal is eminently liable to have its limbs, tail, eyes and jaws bitten
off by other tritons. (27/24. Lessona states that this is so in the paper just
referred to. See also 'The American Naturalist' September 1871 page 579.) Even
with the aquatic salamander the capacity is to a certain extent localised, for
when M. Philipeaux (27/25. 'Comptes Rendus' October 1, 1866 and June 1867.)
extirpated the entire fore limb together with the scapula, the power of
regrowth 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 (27/26. Bonnet 'Oeuvres Hist. Nat.' volume 5 page 294, as quoted by
Prof. Rolleston in his remarkable address to the 36th annual meeting of the
British Medical Association.) 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 (27/27. 'Proc. Boston Soc. of
Nat. Hist.' volume 12 1868-69 page 1.), 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 regrowth, 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 regrowth, 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 (27/28. 'Transact. Linn. Soc.' volume 24 1863 page 62.)
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.
(27/29. 'Parthenogenesis' 1849 pages 25, 26. Prof. Huxley has some excellent
remarks ('Medical Times' 1856 page 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.)
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. (27/30. Prof. J. Reay Greene in
Gunther's 'Record of Zoolog. Lit.' 1865 page 625.) 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 larvae 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 Muller 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 Palaemon 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. (27/31. Fritz Muller 'Fur Darwin' 1864 s. 65, 71. The
highest authority on crustaceans, Prof. Milne-Edwards, insists ('Annal. des
Sci. Nat.' 2nd series Zoolog. tome 3 page 322) on the difference in the
metamorphosis of closely-allied genera.) 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 Medusae 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 Hydroidae any
phase, planuloid, polypoid, or medusoid, may be absent." (27/32. Prof. Allman
'Annals and Mag. of Nat. Hist.' 3rd series volume 13 1864 page 348; Dr. S.
Wright ibid volume 8 1861 page 127. See also page 358 for analogous statements
by Sars.)
According to the belief now generally accepted by our best naturalists, all
the members of the same order or class, for instance, the Medusae 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 (27/33. 'Tissus Vivants' 1866 page 22.), has its
proper life, its autonomy; it can develop and reproduce itself independently
of the adjoining tissues. A great German authority, Virchow (27/34. 'Cellular
Pathology' translated by Dr. Chance 1860 pages 14, 18, 83, 460.), 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. (27/35. Paget 'Surgical Pathology' volume 1 1853 pages 12-14.) 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 (27/36. Ibid page 19.) 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. (27/37. See Prof. Mantegazza's interesting work 'Degli innesti
Animali' etc. Milano 1865 page 51 tab. 3.) The tail of a pig has been grafted
into the middle of its back, and reacquired sensibility. Dr. Ollier (27/38.
'De la Production Artificielle des Os' page 8.) 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 (27/39. Isidore Geoffroy Saint-Hilaire 'Hist. des
Anomalies' tome 2 pages 549, 560, 562; Virchow ibid page 484. Lawson Tait 'The
Pathology of Diseases of the Ovaries' 1874 pages 61, 62.), 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. (27/40. For the most recent classification of cells, see
Ernst Hackel 'Generelle Morpholog.' b. 2 1866 s. 275.) The doctrine of omnis
cellula e cellula is admitted for plants, and widely prevails with respect to
animals. (27/41. Dr. W. Turner 'The Present Aspect of Cellular Pathology'
'Edinburgh Medical Journal' April 1863.) 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
Scrophulariaceae. 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?