natural selection is excluded on the grounds of the extreme complexity
of the changes, and adduces the case of the Irish "elk" with its huge
antlers, and the giraffe with its specially modified structure. He
points out that in either case the conspicuous modification--the
gigantic antlers or the long neck--involves a multitude of changes
affecting many and sometimes distant parts of the body. Not only have
the enormous antlers involved changes in the skull, the bones of the
neck, the muscles, blood-vessels, and nerves of this region, but changes
also in the fore limbs; while the long neck of the giraffe has brought
with it a complete change of gait, the co-ordinated movements of the
hind limbs sharing in the general modification. Mr. Spencer, therefore,
argues that it is difficult to believe that these multitudinous
co-ordinated modifications are the result of fortuitous variations
seized upon by natural selection. For natural selection would have to
wait for the fortunate coincidence of a great number of distinct parts,
all happening to vary just in the particular way required. That natural
selection should seize upon the favourable modification of a particular
part is comprehensible enough; that two organs should coincidently vary
in favourable directions we can understand; that half a dozen parts
should, in a few individuals among the thousands born, by a happy
coincidence, vary each independently in the right way is conceivable;
but that the whole organization should be remodelled by fortunately
coincident and fortuitously favourable variations is not readily
comprehensible. It may be answered--Notwithstanding all this, we know
that such happy coincidences have occurred, for there is the resulting
giraffe. The question, however, is not whether these modifications have
occurred or not, but whether they are due to fortuitous variation alone,
or have been guided by functional use. The argument seems to me to have
weight.[DI]
Still, we should remember that among neuter ants--for example, in the
Sauba ant of South America (_Oecodoma cephalotes_)--there are certain
so-called soldiers with relatively enormous heads and mandibles. The
possession of these parts so inordinately developed must necessitate
many correlated changes. But these cannot be due to inherited use, since
such soldiers are sterile.
Furthermore, according to Professor Weismann, natural selection is
really working, not on the organism at large, but on the germ-plasm
which produces it; and it is conceivable that the variation of one or
more of the few cells in early embryonic life may introduce a great
number of variations in the numerous derivative cells. In explanation of
my meaning, I will quote a paragraph from a paper of Mr. E. B. Poulton's
on "Theories of Heredity."[DJ] "It appears," he says, "that, in some
animals, the great groups of cells are determined by the first division
[of the ovum in the process of cleavage[DK]]; in others, the right and
left sides, or front and hind ends of the body; while the cells giving
rise to the chief groups on each side would then be separated at some
later division. This is not theory, but fact; for Roux has recently
shown that, if one of the products of the first division of the egg of a
frog be destroyed with a hot needle, development is not necessarily
arrested, but, when it proceeds, leads to the formation of an embryo
from which either the right or the left side is absent. When the first
division takes place in another direction, either the hind or the front
half was absent from the embryo which was afterwards produced. After the
next division, when four cells were present, destruction of one produced
an embryo in which one-fourth was absent." Now, it is conceivable that a
single modification or variation of the primitive germ might give rise
to many correlated modifications or variations of the numerous cells
into which it develops; just as an apparently trivial incident in
childhood or youth may modify the whole course of a man's subsequent
life. It is difficult, indeed, to see how this could be effected; to
understand what could be the nature of a modification of the germ which
could lead simultaneously to many favourable variations of bones,
muscles, blood-vessels, and nerves in different parts of the body. This,
however, is a question of the origin of variations; and it is, at any
rate, conceivable that, just as by the extirpation with a hot needle of
one cell of the cleaved frog's ovum all the anterior part of the body
should be absent in development, so by the appropriate modification of
this one cell, or the germinal matter which produced it, all the
anterior part of the body should be appropriately modified.
These considerations, perhaps, somewhat weaken the force of Mr.
Spencer's argument, which is not quite so strong now as it was when the
"Principles of Biology" was published.
(2) We may pass now to the evidence afforded by direct observation and
experiment. There is little enough of it. The best results are, perhaps,
those which have been incidentally reached in the poultry-yard and on
the farm in the breeding of domesticated animals. We have seen that,
under these circumstances, certain parts or organs have very markedly
diminished in size and efficiency; others have as markedly increased. Of
the former, or decrease in size and efficiency, the imbecile ducks with
greatly diminished brains have been already mentioned. Mr. Herbert
Spencer draws attention[DL] to the diminished efficiency in ear-muscles,
giving rise to the drooping ears of many domesticated animals. "Cats in
China, horses in parts of Russia, sheep in Italy and elsewhere, the
guinea-pig formerly in Germany, goats and cattle in India, rabbits,
pigs, and dogs in all long-civilized countries, have dependent
ears."[DM] Since many of these animals are habitually well fed, the
principle of economy of growth seems excluded. Indeed, the ears are
often unusually large; it is only their motor muscles that have dwindled
either relatively or absolutely. If what has been urged above be valid,
panmixia cannot have been operative; since panmixia _per se_ only brings
about regression to mediocrity. If the effects in these two cases,
ducks' brains and dogs' ears, be not due to disuse, we know not at
present to what they are due. In the correlative case of increase by
use, we find it exceedingly difficult to exclude the disturbing effects
of artificial selection. The large and distended udders of cows, the
enhanced egg-laying powers of hens, the fleetness or strength of
different breeds of horses,--all of these have been subjects of
long-continued, assiduous, and careful selection. One cannot be sure
whether use has co-operated or not.
Sufficient has now, I think, been said to show the difficulty of
deciding this question, the need of further observation and discussion,
and the necessity for a receptive rather than a dogmatic attitude; and
sufficient, also, to indicate my reasons for leaning to the view that
use and disuse, long-continued and persistent, may be a factor in
organic evolution.
_The Nature of Variations._
The diversity of the variations which are possible, and which actually
occur in animal life, is so great that it is not easy to sum up in a
short space the nature of variations. Without attempting anything like
an exhaustive classification, we may divide variations into three
classes.
1. _Superficial variations_ in colour, form, etc., not necessarily in
any way correlated with
2. _Organic variations_ in the size, complexity, and efficiency of the
organs of the body;
3. _Reproductive and developmental variations._
Any of these variations, if sufficient in amount and value to determine
the question of elimination or not-elimination, selection or
not-selection, may be seized upon by natural selection.
Our domesticated animals exemplify very fully the superficial variations
which, through man's selection, have in many cases been segregated and
to some extent stereotyped. It is unnecessary to do more than allude to
the variations in form and coloration of dogs, cattle, fowls, and
pigeons. These variations are not _necessarily_ in any way correlated
with any deeper organic variations. They are, however, in many cases so
correlated. For example, the form of the pouter pigeon is correlated
with the increased size of the crop, the length of the beak carries with
it a modification of the tongue, the widely expanded tail of the fantail
carries with it an increase in the size and number of the caudal
vertebræ. And here we might take the whole series of secondary sexual
characters. These and their like may be said to be direct correlations.
But there are also correlations which are seemingly indirect, their
connection being apparently remote. That in pigeons the size of the feet
should vary with the size of the beak; that the length of the wing and
tail feathers should be correlated; that the nakedness of the young
should vary with the future colour of the plumage; that white dogs
should be subject to distemper, and white fowls to the "gapes;" that
white cats with blue eyes should be nearly always deaf;--in these cases
the correlation is indirect. But from the existence of correlation,
whether direct or indirect, it follows that variations seldom come
singly. The organism is so completely a unity that the variation of one
part, even in superficial matters, affects directly or indirectly other
parts.
In the freedom of nature such superficial variations are not so obvious.
But among the invertebrates they are not inconsiderable. The case of
land-snails, already quoted, may again be cited. Taking variations in
banding alone, Mr. Cockerell knows of 252 varieties of _Helix nemoralis_
and 128 of _H. hortensis_. Still, among the wild relatives of our
domestic breeds of animals and birds the superficial variations are
decidedly less marked. And this is partly due to the fact that they are
in a state of far more stable equilibrium than our domestic products,
and partly to the constant elimination of all variants which are thereby
placed at a serious or vital disadvantage. White rats, mice, or small
birds, in temperate regions, would soon be seized upon by hawks and
other enemies. If the eggs and young of the Kentish plover, shown in our
frontispiece, were white or yellowish, like the eggs and young of our
fowls, they would soon be snapped up. The varied protective
resemblances, general and special, have been brought about by the
superficial variations of organisms, and the elimination of those which,
from non-variation or wrong variation, remained conspicuous. We need
only further notice one thing here, namely, that, in the case of special
resemblance to an inorganic object or to another organism, the
variations of the several parts must be very closely, and sometimes
completely, correlated. The correlations, however, need not, perhaps,
have been simultaneous--the resemblance having been gradually perfected
by the filling in of additional touches, first one here, then another
there, and so on.
Concerning "organic variations," little need be said. It is clear that
an organ or limb may vary in size, such variation carrying with it a
correlative variation in power; or it may vary in complexity--the teeth
of the horse tribe, for example, having increased in complexity, while
their limbs have been rendered less complex; or it may vary in
efficiency through the more perfect correlation and co-ordination of its
parts.
The evidence of such variations from actual observation is far less in
amount than that of superficial variations. And this is not to be
wondered at, since in many cases it can only be obtained by careful
anatomical investigation. Nevertheless, anatomists, both human and
comparative, are agreed that such variations do occur. And no one can
examine such a collection as that of the Royal College of Surgeons
without acknowledging the fact.
Thirdly, "reproductive and developmental variations" are of very great
importance. The following are among the more important modifications
which may occur in the animal kingdom.
1. Variations in the mode of reproduction, sexual or asexual.
2. Variations in the mode of fertilization.
3. Variations in the number of fertilized ova produced.
4. Variations in the amount of food-yolk and in the way in which it is
supplied.
5. Variations in the time occupied in development.
6. Variations in the time at which reproduction commences.
7. Variations in the duration and amount of parental protection and
fosterage.
8. Variations in the period at which secondary sexual characters and the
maximum efficiency of the several organs is reached.
It is impossible here to discuss these modes of variation _seriatim_. I
shall therefore content myself with but a few remarks on the importance
of protection and fosterage. It is not too much to say that, without
fosterage and protection, the higher forms of evolution would be
impossible. If you are to have a highly evolved form, you must allow
time for its evolution from the egg; and that development may go on
without let or hindrance, you must supply the organism with food and
lighten the labour of self-defence. Most of the higher organisms are
slow in coming to maturity, passing through stages when they are
helpless and, if left to themselves, would inevitably fall a prey to
enemies.
In those animals in which the system of fosterage and protection has not
been developed a great number of fertilized ova are produced, only a few
of which come to maturity. It might be suggested that this is surely an
advantage, since the greater the number produced the greater the chances
of favourable variations taking place. But it has before been pointed
out that these great numbers are decimated, and more than decimated, not
by elimination, but by indiscriminate destruction; embryos, good, bad,
and indifferent, being alike gobbled up by those who had learnt the
secret of fostering their young. The alternative has been between
producing great numbers[DN] of embryos which soon fend for themselves,
and a few young who are adequately provided for during development. And
the latter have proved the winners in life's race. If we compare two
flat-fishes belonging to very different groups, the contrast here
indicated will be readily seen. The skate is a member of the shark
tribe, flattened symmetrically from above downwards. It lays, perhaps,
eighty to a hundred eggs. Each of these is large, and has a rich supply
of nutritive food-yolk. Each is also protected by a horny case with
pointed corners--the so-called sea-purse of seaside visitors. These are
committed by the skate to the deep, and are not further cared for. But
the abundant supply of food-yolk gives the little skate which emerges a
good start in life. On the other hand, the turbot, one of the bony
fishes, flattened from side to side with an asymmetrical head, lays
several millions of eggs, which float freely in the open sea. These are
minute and glassy, and not more than one-thirtieth of an inch in
diameter. When the fishes are hatched, they are not more than about
one-fifth of an inch in length. The slender stock of food-yolk is soon
used up, and henceforth the little turbot (at present more like a
stump-nosed eel than a turbot) has to get its own living. Hundreds of
thousands of them are eaten by other fishes.
Or, if we compare such different vertebrates as a frog, a sparrow, and a
mouse, we find that the frog produces a considerable number of
fertilized ova, though few in comparison with the turbot, each provided
with a small store of food-yolk. The tiny tadpoles very soon have to
obtain their own food and run all the risks of destruction. Few survive.
The sparrow lays a few eggs; but each is supplied with a large store of
food-yolk, sufficient to meet its developmental needs until, under the
fostering influence of maternal warmth, it is hatched. Even on emerging
from the eggs, the callow fledglings enjoy for a while parental
protection and fosterage, and, when sent forth into the world, are very
fairly equipped for life's struggle. The mouse produces minute eggs with
little or no food-yolk; but they undergo development within the womb of
the mother, and are supplied with nutrient fluids elaborated within the
maternal organism. Even when born, they are cherished for a while and
supplied with food-milk by the mother.
The higher stages of this process involve a mental element, and are
developed under the auspices of intelligence or instinct. But the lower
stages, the supply of food-yolk and intra-uterine protection, are purely
organic. A hen cannot by instinctive or intelligent forethought increase
the amount of food-yolk stored up in the ovum, any more than the lily,
which, by an analogous process, stores up in its bulb during one year
material for the best part of next year's growth, can increase this
store by a mental process.
It cannot therefore be questioned that variations in the amount of
capital with which an embryo is provided in generation would very
materially affect its chances of escaping elimination by physical
circumstances, by enemies, and by competition.
Nor can it be questioned that variations in the time occupied in
reaching maturity would, other things equal, not a little affect the
chances of success of an organism in the competition of life. Hence we
have the phenomena of what may be termed acceleration and retardation in
development. These terms have, however, been used by American
zoologists, notably Professors Hyatt and Cope, in a somewhat different
and wider sense; for they include not merely time-changes, but also the
loss of old characters or the acquisition of new characters. "It is
evident," says Professor Cope, "that the animal which adds something to
its structure which its parents did not possess has grown more than
they; while that which does not attain to all the characteristics of its
ancestors has grown less than they." "If the embryonic form be the
parent, the advanced descendant is produced by an increased rate of
growth, which phenomenon is called 'acceleration'; but if the embryonic
type be the offspring, then its failure to attain the condition of the
parent is due to the supervention of a slower rate of growth; to this
phenomenon the term 'retardation' is applied." "I believe that this is
the simplest mode of stating and explaining the law of variation: that
some forms acquire something which their parents did not possess; and
that those which acquire something additional have to pass through more
numerous stages than their ancestors; and those which lose something
pass through fewer stages than their ancestors; and these processes are
expressed by the terms 'acceleration' and 'retardation.'"[DO]
It is clear, however, that we have here something more than acceleration
and retardation of development in the ordinary sense of these words. It
would be, therefore, more convenient to use the term "acceleration" for
the condensation of _the same series_ of developmental changes into a
shorter period of time; "retardation" for the lengthening of the period
in which _the same series_ of changes are effected; and "arrested
development" for those cases in which the young are born in an immature
or embryonic condition. Whether there is any distinct tendency, worthy
of formulation as a law, for organisms to acquire, as a result of
protracted embryonic development, definite characteristics which their
ancestors did not possess, I think very questionable. If so, this will
fall under the head of the origin of variations.
That acceleration, in the sense in which I have used the term, does
occur as a variation is well known. "With our highly improved breeds of
all kinds," says Darwin,[DP] "the periods of maturity and reproduction
have advanced with respect to the age of the animal; and in
correspondence with this, the teeth are now developed earlier than
formerly, so that, to the surprise of agriculturalists, the ancient
rules for judging of the age of an animal by the state of its teeth are
no longer trustworthy." "Disease is apt to come on earlier in the child
than in the parent; the exceptions in the other direction being very
much rarer."[DQ] Professor Weismann contends that the time of
reproduction has been accelerated through natural selection, since the
shorter the time before reproduction, the less the number of possible
accidents. We may, perhaps, see in the curious cases of reproduction
during an otherwise immature condition, extreme instances of
acceleration. The axolotl habitually reproduces in the gilled, or
immature condition. Some species of insects reproduce before they
complete their metamorphoses. And the females of certain beetles
(_Phengodini_) are described by Professor Riley as larviform.[DR]
Precocity is variation in the direction of acceleration, and that
condensed development which is familiar in the embryos of so many of the
higher animals may be regarded as the result of variations constantly
tending in the same direction. That there are fewer examples of
retardation is probably due to the fact that nature has constantly
favoured those that can do the same work equally well in a shorter time
than their neighbours. But there can be no doubt that, accompanying that
fosterage and protection which is of such marked import in the higher
animals, there is also much retardation. And as bearing upon the
supposed law of variation as formulated by Messrs. Hyatt and Cope, it
should be noted that this retardation or _decreased_ rate of growth
leads to the production of the more advanced descendant.
_The Inheritance of Variations._
Given the occurrence of variations in certain individuals of a species,
we have the alternative logical possibilities of their being inherited
or their not being inherited. The latter alternative seems at first
sight to be in contradiction to the law of persistence. Sir Henry
Holland, seeing this, remarked that the real subject of surprise is, not
that a character should be inherited, but that any should ever fail to
be inherited.[DS] Intercrossing may diminish a character, and sooner or
later practically obliterate it: annihilate it at once and in the first
generation it cannot. This logical view, however, ceases to be binding
if we admit, with Professor Weismann, that variations may be produced in
the body without in any way affecting the germ. It is also vitally
affected if we believe that the hen does not produce the egg, though she
may, perhaps, modify the eggs inside her; for the modification of the
hen (i.e. the variety in question) may not be of the right nature or of
sufficient strength to impress itself upon the germinal matter of the
egg. We may at once admit, then, that acquired variations need not be
inherited.
Passing to innate variations--variations, that is to say, which are the
outcome of normal development from the fertilized ovum--must they be
inherited, at any rate, in some degree? It seems to me that they must,
on the hypothesis that sexual generation involves simply the blending or
commingling of the characters handed on in the ovum or the sperm. The
only cases where this would _apparently_ fail to hold good would be
where the ovum and the sperm handed on exactly opposite tendencies--a
variation in excess contributed by the male precisely counterbalancing a
variation in the opposite direction contributed by the female parent.
Even here the tendency is inherited, though it is counterbalanced. On
the hypothesis of "organic combination" before alluded to (p. 150),
variations might, however, in the union of ovum and sperm, be not only
neutralized, but augmented. If the variation be, so to speak, a definite
organic compound resulting from a fortunate combination of characters in
ovum and sperm, it might either fail altogether, or be repeated in an
enfeebled form, or augmented in the offspring, according as the new
conditions of combination were unfavourable or favourable.
Whether innate variations ever actually fail to be inherited, even in an
enfeebled form, it is very difficult to say; for if this, that, or the
other variation fail to be thus inherited, it is difficult to exclude
the possibility of its being an acquired variation not truly innate.
Certainly variations seem sometimes to appear in one generation, and not
to be inherited at all. And, as we have seen, Mr. Romanes appeals to a
gradual failure of heredity, apart from intercrossing, to explain the
diminution of disused organs.
That a variation strongly developed in both parents is apt to be
augmented in the offspring is commonly believed by breeders. Darwin was
assured that to get a good jonquil-coloured canary it does not answer to
pair two jonquils, as the colour then comes out too strong, or is even
brown. Moreover,[DT] "if two crested canaries are paired, the young
birds rarely inherit this character; for in crested birds a narrow space
of bare skin is left on the back of the head, where the feathers are
upturned to form the crest, and, when both parents are thus
characterized, the bareness becomes excessive, and the crest itself
fails to be developed."
On the whole, it would seem that variations may either be neutralized or
augmented in inheritance; but the determining causes are not well
understood.
Another fact to be noticed with regard to the inheritance of variations
is that some characters blend in the offspring, while others apparently
fail to do so. Mr. Francis Galton,[DU] speaking of human characters,
gives the colour of the skin as an instance of the former, that of the
eyes as an example of the latter. If a negro marries a white woman, the
offspring are mulattoes. But the children of a light-eyed father and a
dark-eyed mother are either light-eyed or dark-eyed. Their eyes do not
present a blended tint. Among animals the colour of the hair or feathers
is often a mean or blended tint; but not always. Darwin gives the case
of the pairing of grey and white mice, the offspring of which are not
whitish-grey, but piebald. If you cross a white and a black game bird,
the offspring are either black or white, neither grey nor piebald. Sir
R. Heron crossed white, black, brown, and fawn-coloured Angora rabbits,
and never once got these colours mingled in the same animal, but often
all four colours in the same litter. He also crossed "solid-hoofed" and
ordinary pigs. The offspring did not possess all four hoofs in an
intermediate condition; but two feet were furnished with properly
divided and two with united hoofs.[DV] Professor Eimer[DW] has noticed
that, in the crossing of striped and unstriped varieties of the garden
snail, _Helix hortensis_, the offspring are either striped or unstriped,
not in an intermediate or faintly striped condition.
These facts are of no little importance. They tend to minimize, for some
characters at least, the effects of intercrossing. The variations which
present this trait may be likened to stable organic compounds, which may
be inherited or not inherited, but which cannot be watered down by
admixture and intercrossing. It is well known[DX] that, in 1791, a
ram-lamb was born in Massachusetts, with short, crooked legs and a long
back, like a turn-spit dog. From this one lamb[DY] the _otter_, or
_ancon_, breed was raised. When sheep of this breed were crossed with
other breeds, the lambs, with rare exceptions, perfectly resembled one
parent or the other. Of twin lambs, even, one has been found to resemble
one parent, and the second the other. All that the breeder has to do is
to eliminate those which do not possess the required character. And very
rarely do the lambs of ancon parents fail to be true-bred.
Now, it can scarcely fail that such sports occur in nature. And if they
are stable compounds, they will not be readily swamped by intercrossing.
It only requires some further isolation to convert the sporting
individuals into a distinct and separate variety. Now, Darwin tells us
that the ancons have been observed to keep together, separating
themselves from the rest of the flock when put into enclosures with
other sheep. Here, then, we have preferential mating as the further
isolating factor. I feel disposed, therefore, to agree with Mr. Galton
when he says,[DZ] "The theory of natural selection might dispense with a
restriction for which it is difficult to see either the need or the
justification, namely, that the course of evolution always proceeds by
steps that are severally minute, and that become effective only through
accumulation. That the steps _may_ be small, and that they _must_ be
small, are very different views; it is only to the latter that I object,
and only when the indefinite word 'small' is used in the sense of
'barely discernible,' or as small as compared with such large sports as
are known to have been the origins of new races."
Connected, perhaps, with the phenomena we have just been considering is
that of _prepotency_.[EA] It is found that, when two individuals of the
same race or of different races are crossed, one has a preponderant
influence in determining the character of the offspring. Thus the famous
bull Favourite is believed to have had a prepotent influence on the
short-horn race; and the improved short-horns possess great power in
impressing their likeness on other breeds. The phenomena are in some
respects curiously variable. In fowls, silkiness of feathers seems to be
at once bred out by intercrossing between silk-fowl and any other breed.
But in the silky variety of the fan-tail pigeon this character seems
prepotent; for, when the variety is crossed with any other small-sized
race, the silkiness is invariably transmitted. One may fairly suppose
that prepotent characters have unusual stability; but to what causes
this stability is due we are at present ignorant.
Lastly, we have to consider the phenomenon of _latency_. This is the
lying hid of characters and their subsequent emergence. We may
distinguish three forms of latency.
1. Where characters lie hid till a certain period of life, and then
normally emerge.
2. Where the characters normally lie hid throughout life, but are, under
certain circumstances, abnormally developed.
3. Where the characters lie hid throughout life, but appear in the
offspring or (sometimes distant) descendants.
Latency is often closely connected with correlated variations. Secondary
sexual characters, for example, are correlated with the functional
maturity or activity of the reproductive organs. They therefore lie hid
until these organs are mature and ready for activity. When they are
restricted to the male, they normally remain latent throughout the life
of the female, but reappear in her male offspring. Under abnormal
conditions, such as the removal of the essentially male organs, the
secondary sexual characters correlated with them do not appear, or
appear in a lessened and modified form. The males may even, under such
circumstances, acquire female characters. Thus capons take to sitting,
and will bring up young chickens. Conversely, females which have lost
their ovaries through disease or from other causes sometimes acquire
secondary sexual characters proper to the male. Characters thus normally
latent abnormally emerge. Mr. Bland Sutton[EB] gives a case of a hen
golden pheasant which "presented the resplendent dress of the cock, but
her plumage was not quite so brilliant; she had no spurs, and the iris
was not encircled by the ring of white so conspicuous in the male." Her
ovary was no larger than a split pea.
A curious instance of latent characters correlated with sex is seen in
hive bees. The worker bee differs from the female in the rudimentary
condition of the sexual organs, in size and form, and in the higher
development of the sense-organs. But it is well known that, if a very
young worker grub be fed on "royal jelly," she will develop into a
perfect queen. Not only are the sexual organs stimulated to increased
growth and functional activity, but the correlated size and condition of
the sense-organs are likewise acquired. The characters of queen and
worker are latent in the grub. According to the nature of the food it
receives, the one set of characters or the other emerges. Professor
Yung's tadpoles and Mrs. Treat's butterflies (_ante_, p. 59) afford
similar instances.
We come now to those cases of latency in which this obvious correlation
does not occur. They afford examples of reversion to more or less remote
ancestral characters. In some cases the cause of such reversion--such
unexpected emergence of characters, which have remained latent through
several, perhaps many, generations--is quite unknown. In others, at any
rate among domesticated animals, the determining condition of such
reversion is the crossing of distinct breeds.
Darwin gives[EC] an instance of reversion, on the authority of Mr. R.
Walker. He bought a black bull, the son of a black cow with white legs,
white belly, and part of the tail white; and in 1870 a calf, the
gr-gr-gr-gr-grandchild of this cow, was born, coloured in the same very
peculiar manner, all the intermediate offspring having been black. In
man partial reversions are not infrequent. An additional pair of lumbar
ribs is sometimes developed, and in such cases the fan-shaped tendons
which are normally connected with the transverse processes of the
vertebræ are replaced by functional levator muscles. Since it is
probable that the ancestor of man had more than the twelve pairs of ribs
that are normally present in the human species, we may, perhaps, fairly
regard the supernumerary rib as a reversion. But it may be a new sport
on old lines.
The occasional occurrence in Scotland of red grouse with a large amount
of white in the winter plumage, especially on the under parts, is justly
regarded by Mr. Wallace[ED] as a good example of reversion or latency in
wild birds. There can be little doubt that, as he suggests, the Scotch
red grouse is derived from a form which, like the wide-ranging willow
grouse, has white winter plumage. During the glacial epoch this would be
an advantage. "But when the cold passed away, and our islands became
permanently separated from the mainland, with a mild and equable
climate, and very little snow in winter, the change to white at that
season became hurtful, rendering the birds more conspicuous, instead of
serving as a means of concealment." The red grouse has lost its white
winter dress; but occasional reversions point to the ancestral habit.
That crossing tends to produce reversion is a fact familiar to breeders
and fanciers, and one which is emphasized by Darwin. When pigeons are
crossed, there is a strong tendency to revert to the slatey-blue tint
and black bars of the ancestral rock-pigeon. There is always a tendency
in sheep to revert to a black colour, and this tendency is emphasized
when different breeds are crossed. The crossing of the several equine
species (horse, ass, etc.) "tends in a marked manner to cause stripes to
appear on various parts of the body, especially on the legs," and this
_may be_ a reversion to the condition of a striped and zebra-like
ancestor. Professor Jaeger described a good case with pigs. "He crossed
the Japanese, or masked breed, with the common German breed, and the
offspring were intermediate in character. He then recrossed one of these
mongrels with a pure Japanese, and in the litter thus produced one of
the young resembled in all its characters a wild pig; it had a long
snout and upright ears, and was striped on the back. It should be borne
in mind that the young of the Japanese breed are not striped, and that
they have a short muzzle and ears remarkably dependent."[EE] Darwin
crossed a black Spanish cock with a white silk hen. One of the offspring
almost exactly resembled the _Gallus bankiva_, the remote ancestor of
the parents.
Such cases would seem to show that in our domestic breeds ancestral
traits lie latent. The crossing of distinct varieties may either
neutralize the variations artificially selected, and thus allow the
ancestral characters which have been masked by them to reappear; or they
may allow the elements of the ancestral traits, long held apart in
separate breeds by domestication, to recombine with the consequent
emergence of the normal characters of the wild species. But, in truth,
any attempted explanations of the facts are little better than
guess-work. There are the facts. And the importance of crossing as a
determining condition in domesticated animals should make us cautious in
applying reversion, as it occurs in such cases, to wild species which
live under more stable conditions where crossing is of rare occurrence.
_The Origin of Variations._
The subject of the origin of variations is a difficult one, one
concerning which comparatively little is known, and one on which I am
not able to throw much light.
Taking a simple animal cell as our starting-point, we have already seen
that it performs, in primitive fashion, certain elementary and essential
protoplasmic activities, and gives rise to certain products of
cell-life. In the metazoa, which are co-ordinated aggregates of animal
cells, together with some of their products, there is seen a division of
labour and a differentiation of structure among the cells. We see, then,
that variation among these related cells has led to differences in size,
in form, in transparency, and in function; while the cell-products have
been differentiated into those which are of lifelong value, such as
bone, cartilage, connective tissue, horn, chitin, etc., together with a
variety of colouring matters; those which are of temporary value, such
as the digestive secretions, fat, etc.; and those which are valueless or
noxious, such as carbonic acid gas and urea, which are excreted as soon
as possible. Here are already a number of important and fundamental
variations to be accounted for.
Let us notice that, wide as the variations are, they are to a large
extent hedged in by physical, chemical, and organic limitations. We have
already seen that the size of cells is to a large extent limited,
because during growth mass tends to outrun surface; and because, while
disruptive changes occur throughout the mass, nutriment and oxygen must
be absorbed by the surface. This is a physical limitation. Since the
products of cell-life and cell-activity are chemical products, it is
clear that they can only be produced under the fixed limitations of
chemical combination; and though in organic products these limitations
are not so rigid as among inorganic substances, still that there are
limitations no chemist is likely to question. The organic limitations
are to the varied, but not very numerous, modes of protoplasmic
activity.
Probably, even at the threshold of metazoan life, such variations did
not affect only individual cells, but rather groups of cells. In
other words, the differentiation was at once and primarily a
tissue-differentiation. What do we know, however, about the primitive
tissue-differentiation of the earliest metazoa? Hardly anything. We may
fairly suppose that the first marked difference to appear was that
between the outside and the inside. In the formation of an embryo this
is the first differentiation we notice. From the beginning of
segmentation or, in any case, very early, the outer-layer cells become
marked off from the inner-layer cells. The next step was, perhaps, the
formation of the mid-layer between the outer and inner. But how further
differentiations were effected we really do not know, though we may
guess a little. This, perhaps, we may fairly surmise--that fresh
differentiations presupposed previous differentiations, and formed the
basis of yet further differentiations. Thus calcified cartilage
presupposes cartilage, and leads up to the formation of true bone. In
all this, however, we are very much in the dark. We can watch, always
with fresh wonder, the genesis of tissues in the development of the
embryo; but we do not at present know much of the mode of their
primitive genesis in the early days of organic evolution: how can we,
then, pretend to understand their origins?
If we speculate at all on the matter, we are led to the view that the
variations must be primarily due to the differential incidence of
mechanical stresses and physical or chemical influences. It may be
admitted that this is little more than saying that they are due to some
physical cause. Still, this at least may be taken as certain for what it
is worth--that the primitive tissue-differentiations are due to physical
or chemical influences, direct or indirect, on the protoplasm of the
cell. Here is one mode of the origin of variations.
I do not wish to reopen the question whether these variations originate
in the germ or in the body. I content myself with indicating the
difference, from this standpoint, between the two views. Take, for
example, the end-organs of the special senses, which respond explosively
to physical influences in ways we shall have to consider more fully in
the next chapter. If we hold that variations originating in the body may
be transmitted through the germ to the offspring, then we may say that
these variations are the direct result of the incidence of the physical
or molecular vibrations on the protoplasm. But if we believe, with
Professor Weismann, that all variations originate in the germ, then the
variations in the end-organs of the special senses, fitting them to be
the recipients of special modes of influence, result from physical
effects upon the germ of purely fortuitous origin, that is to say,
wholly unrelated to the end in view. The rods and cones of the retina
are due to purely chance variations, impressed by some chemical or
physical causes completely unknown on the germinal protoplasmic
substance. Those individuals which did not have these chance variations
have been eliminated. It matters not that the rods and cones are
believed to have reached their present excellence through many
intermediate steps from much simpler beginnings. The fact remains that
the origin of all these step-like variations was fortuitous, and not in
any way the direct outcome of the physical influences which their
products, the rods and cones, have become fitted to receive. I am not at
present prepared to accept this theory of the germinal origin of all
tissue-variations.
Whether use and disuse are to be regarded as sources of origin of
variations is, again, a matter in which there is wide difference of
opinion. But if we admit that any variations can take their origin in
the body (as distinguished from the germ), then there is no _à priori_
reason for rejecting use and disuse as factors. As such, we are, I
think, justified, in the present state of our knowledge, in reckoning
them, at all events, provisionally.
It is clear, however, that they are a proximate, not an ultimate, source
of origin. I mean that the structures must be there before they can be
either strengthened or weakened by use or disuse. They are at most a
source of positive or negative variations of existing structures. They
cannot be a direct source of origin of superficial variations. Gain or
loss of colour; form-variations not correlated with organic
variations;--these cannot be directly due to use or disuse. It is in the
nervous and muscular systems and the glandular organs that use and
disuse are mainly operative. When, however, organs are brought into
relation, or fail to be brought into relation, to their appropriate
stimuli, we speak of this, too, as use and disuse. We say, for example,
that persistent disuse may impair the essential tissues of the recipient
end-organs of the special senses, implying that these tissues require to
be brought into continued relation to the appropriate stimuli in order
that their efficiency be maintained. So, too, we say that the epidermis
is thickened by use, meaning that it is brought into relation with
certain mechanical stresses. Through correlation, too, the effects of
use and disuse may be widespread. Thus increase in the size of a group
of muscles may be correlated with increase in the size of the bones to
which they are in relation. In fact, so knit together and co-ordinated
is the organism into a unity, it is probable that hardly any variation
could take place through use or disuse without modifying to some extent
the whole organic being.
Once more, let it be clearly remembered that a large and important
school of zoologists reject altogether use or disuse as a factor in
variation. They believe that those germs are selected through natural
selection in which there is an increased tendency to use or disuse of
certain organs. In this, however, we are all agreed. The real question
is what is the source of origin of this tendency. On the view of
germinal origin, we are forced back on unknown physical or chemical
influences in no wise related in origin (though, of course, related in
result) with the use or disuse to which they give rise.
So far the main distinction between the two biological schools seems to
be that the one, placing the origin of variation in the body-tissues,
regards the variations as evoked in direct reaction to physical or
chemical influences; while the other, placing the origin of variation in
the germ, regards the variations as of fortuitous origin.
I do not use the phrase, "of fortuitous origin," as in any sense
discrediting the theory. I am not attempting the cheap artifice of
damning a view that does not happen to be my own with a phrase or a
nickname. And I therefore hasten to point out what variations I do
believe to have had a fortuitous origin. The phrase is often
misunderstood, and they will serve to explain its meaning.
If the reader will kindly refer to the tables of variations in the bats'
wings (Figs. 14-17), he will see that there are a great number of bones
which vary in length and vary independently. And if he will also refer
to Fig. 18, in which seven species of bats are compared, he will see
that the differences arise from the increased length of one set of bones
in one species and another set of bones in another species. Now, let us
suppose that the long, swallow-like wing of the noctule, a high flyer
with rapid wing-strokes, that catches insects in full flight, and the
broad wings of the horse-shoe, a low flyer, flapping slowly, and, at any
rate, sometimes catching insects on the ground, and covering them with
its wings as with a net; let us suppose, I say, that to each species its
special form of wing is an advantage. Among thousands of independent
variations in the lengths of the bones there would be occasional
combinations of variations, giving either increased length or increased
breadth to the wing. In the noctule, the former would tend to be
selected; in the horse-shoe, the latter. Thus the wing of the noctule
would be lengthened, and that of the horse-shoe broadened, through the
selection of fortuitous combinations of variations which chanced to be
favourable. Now, each individual bone-variation is, we believe, due to
some special cause; but the fortunate combination is fortuitous, due to
what we term "mere chance."
Darwin believed that chance, in this sense, played a very important part
in the origin of those favourable variations for which, as he said,
natural selection is constantly and unceasingly on the watch. And there
can be little question that Darwin was right.
We must now consider very briefly some of the proximate causes of
variations. In most of these cases we cannot hope to unravel the nexus
of causation. When a plexus of environing circumstances acts upon a
highly organized living animal, the most we can do in the present state
of knowledge is to note--we cannot hope to explain--the effects
produced.
All readers of Darwin's works know well how insistent he was that the
nature of the organism is more important than the nature of the
environing conditions. "The organization or constitution of the being
which is acted on," he says,[EF] "is generally a much more important
element than the nature of the changed conditions in determining the
nature of the variation." And, again,[EG] "We are thus driven to
conclude that in most cases the conditions of life play a subordinate
part in causing any particular modification; like that which a spark
plays when a mass of combustible matter bursts into flame--the nature of
the flame depending on the combustible matter, and not on the spark."
Recent investigations have certainly not lessened the force of Darwin's
contention. From which there follows the corollary that the vital
condition of the organism is a fact of importance. Darwin was led to
believe that among domesticated animals and plants good nutritive
conditions were favourable to variation. "Of all the causes which induce
variability," he says,[EH] "excess of food, whether or not changed in
nature, is probably the most powerful." Darwin also held that the male
is more variable than the female--a view that has been especially
emphasized by Professor W. K. Brooks. Mr. Wallace, as we have already
seen, regards the secondary sexual characters of male birds as the
direct outcome of superabundant health and vigour. "There is," he
says,[EI] "in the adult male a surplus of strength, vitality, and
growth-power which is able to expend itself in this way without injury."
And Messrs. Geddes and Thomson contend[EJ] that "brilliancy of colour,
exuberance of hair and feathers, activity of scent-glands, and even the
development of weapons, are in origin and development outcrops of a male
as opposed to a female constitution."
There is, I think, much truth in these several views thus brought into
apposition. Vigour and vitality, predominant activity and the consequent
disruptive changes, with their abundant by-products utilized in
luxuriant outgrowths and brilliant colours, are probably important
sources of variation. They afford the material for natural selection and
sexual selection to deal with. These guide the variations in specific
directions. For I am not prepared to press the theory of organic
combination so far as to believe that this alone has served to give
definiteness to the specific distinctions between secondary sexual
characters, though it may have been to some extent a co-operating
factor. This, however, is a question apart from that of origin.
Superabundant vigour may well, I think, have been a source of _origin_,
not only of secondary sexual characters, but of many other forms of
variation.
And while these forms of variation may be the special prerogative of the
male, we may perhaps see, in superabundant female vigour, a not less
important source of developmental and embryonic variations in the
offspring. The characteristic selfishness of the male applies his
surplus vitality to the adornment of his own person; the characteristic
self-sacrifice of the mother applies her surplus vitality to the good of
her child. Here we may have the source and origin of those variations in
the direction of fosterage and protection which we have seen to have
such important and far-reaching consequences in the development of
organic life. The storage of yolk in the ovum, the incubation of heavily
yolked eggs, the self-sacrificing development in the womb, the
elaboration of a supply of food-milk,--all these and other forms of
fosterage may well have been the outcome of superabundant female vigour,
the advantages of which are thus conferred upon the offspring.
We may now proceed to note, always remembering the paramount importance
of the organism, some of the effects produced by changes in the
environment.
The most striking and noteworthy feature about the effects of changes of
climate and moisture, changes of salinity of the water in aquatic
organisms, and changes of food-stuff, is that, when they produce any
effect at all, they give rise to _definite_ variations. Only one or two
examples of each can here be cited. Mr. Merrifield,[EK] experimenting
with moths (_Selenia illunaria_ and _S. illustraria_), finds that the
variations of temperature to which the pupa, and apparently also the
larva, are subjected tend to produce "very striking differences in the
moths." On the whole, cold "has a tendency, operating possibly by
retardation, to produce or develop a darker hue in the perfect insect;
if so, it may, perhaps, throw some light on the mechanism so often
remarked in north-country examples of widely distributed moths." Mr.
Cockerell[EL] regards moisture as the determining condition of a certain
phase of melanism, especially among Lepidoptera. The same author states
that the snail "_Helix nemoralis_ was introduced from Europe into
Lexington, Virginia, a few years ago. Under the new conditions it varied
more than I have ever known it to do elsewhere, and up to the present
date (1890) 125 varieties have been discovered there. Of these, no less
than 67 are new, and unknown in Europe, the native country of the
species." The effects of the salinity of the water on the brine-shrimp
_Artemia_ have already been mentioned. One species with certain
characteristics was transformed into another species with other
characteristics by gradually altering the saltness of the water. So,
too, in the matter of food, the effects of feeding the caterpillars of a
Texan species of _Saturnia_ on a new food-plant were so marked that the
moths which emerged were reckoned by entomologists as a new species.
The point, I repeat, to be especially noted about these cases and others
which might be cited,[EM] is that the variation produced is a definite
variation. Very probably it is generally, or perhaps always, produced in
the embryonic or larval period of life. In some cases the variation
seems to be transmissible, though definite and satisfactory proofs of
this are certainly wanting. Still, we may say that if the changed
conditions be maintained, the resulting variation will also be
maintained. Under these conditions, at least, the variation is a stable
one. It is probable that, apart from preferential mating, the varieties
thus produced will tend to breed together rather than to be crossed with
the parent form or varieties living under different conditions. In this
way varieties may sometimes arise by definite and perhaps considerable
leaps under the influence of changed conditions. We must not run the
adage, _Natura nil facit per saltum_, too hard, nor interpret _saltum_
in too narrow a sense.
It is true, and we may repeat the statement of the fact for the sake of
emphasis, that we do not know how or why this or that particular
variation should result from this or that change of climate,
environment, or food-stuff; nor do we know why certain variations (such
as that which produced the ancon breed of sheep) should be stable, while
other variations are peculiarly unstable. But in this we are not worse
off than we are in the study of inorganic nature. We do not know why
calcite should crystallize in any particular one of its numerous
varieties of crystalline form; we do not know why some of these are more
stable than others. We may be able to point to some of the conditions,
but we cannot be said to understand why arragonite should be produced
under some circumstances, calcite under others; or why the same
constituents should assume the form of augite in some rocks, and
hornblende in other rocks. We are hedged in by ignorance; and perhaps
one of our chief dangers, becoming with some people a besetting sin, is
that of pretending to know more than we are at present in a position to
know. Our very analogies by which we endeavour to make clear our meaning
may often seem to imply an unwarrantable assumption of knowledge.
In the last chapter I used the term "organic combination," and drew a
chemical analogy. I wished to indicate the particularity and the
stability of certain variations, and the possibility of new departures
through new combinations of variations, the new departure not being
necessarily anything like a mean between the combining variations.[EN] I
trust that this will not be misunderstood as a new chemico-physical
theory of organic forms. I have some fear lest I should be represented
as maintaining that a giraffe or a peacock is a definite organic
compound, with its proper organic form, in exactly the same way as a
rhombohedron of calcite or a rhombic dodecahedron of garnet is a
definite chemical compound, with its proper crystalline form. All that
the analogy is intended to convey is that variations seem, under certain
circumstances, to be definite and stable, and may possibly combine
rather than commingle.
_Summary and Conclusion._
It only remains to bring this chapter to a close with a few words of
summary and conclusion.
The diversity of animal life must first be grasped. We believe that this
diversity is the result of a process or processes of evolution.
Evolution is the term applied to continuity of development. It involves
adaptation; and adaptation to an unchanging environment may become more
and more perfect. But the environment to which organisms are adapted
also changes. Where the change is in the direction of complexity, we
have elaboration; where it is in the direction of simplicity, we have
degeneration. Of these elaboration is the more important. It involves
both a tendency to differentiation giving rise to individuality, and a
tendency to integration giving rise to association. Continued
elaboration is progress; and this is opposed to degeneration.
The factors of evolution fall under two heads--origin and guidance. The
origin of variations lies in mechanical stresses, and chemical or
physical influences. Whether these act on the body (and are transmitted
by inheritance) or only on the germ, is a question which divides
biologists into two schools. In the latter case all variations are
fortuitous; in the former the development of tissue-variations has been
in direct response to the physical or chemical influences. There are,
however, in any case fortuitous combinations of variations.
Whether use and disuse are factors of origin is also a debatable point.
Those who believe that physical influences on the body are transmissible
believe also that the effects of use and disuse are transmissible.
The vital vigour of the organism is a determining condition of
importance. The vital vigour of males has favoured the origin of
secondary sexual characters; that of females, the fostering and
protection of young, and therefore the development in them of vital
vigour.
The almost universally admitted factor in guidance is natural selection.
But we must be careful not to use it as a mere formula.
Whether sexual selection is also a factor is still a matter of opinion.
Without it the specific character and constancy of secondary sexual
features are at present unexplained. If inherited use and disuse are
admitted as factors in origin, they must also be admitted as important
factors in guidance.
Questions of origin and guidance should, so far as is possible, be kept
distinct. These terms, however, apply to the origin and guidance of
variations. In the origin of species guidance is a factor, no doubt a
most important factor. The title of Darwin's great work was, therefore,
perfectly legitimate. And those who say that natural selection plays no
part in the origin of species are, therefore, undoubtedly in error.
NOTES
[CI] It is beyond the scope of this book to give the _evidences_ of
evolution. Such evidence from embryology, from distribution, and
from palæontology, is now abundant. For palæontological evidence,
see Nicholson's "Manual of Palæontology," 3rd edit., especially
the second volume on "Vertebrates," by R. Lydekker.
[CJ] Weismann, "Essays on Heredity," p. 24.
[CK] Ibid. p. 140.
[CL] Weismann, "Essays on Heredity," p. 90.
[CM] Ibid. p. 292. See also a discussion in _Nature_, in which Mr.
Romanes and Professor Ray Lankester took part, beginning vol. xli.
p. 437.
[CN] Weismann, "Essay on Heredity," p. 140.
[CO] "Origin of Species," p. 110.
[CP] With regard to blind cave-fish, Professor Ray Lankester has
suggested that some selection has been effected. Those animals
whose sight-sensitiveness enabled them to detect a glimmer of
light would escape to the exterior, leaving those with
congenitally weak sight to remain and procreate in the darkness of
the cave.
[CQ] Darwin, "Descent of Man," pt. ii. chap. viii.
[CR] "Darwinism," chap. x.
[CS] "Darwinism," p. 295. Messrs. Geddes and Thomson, "The Evolution of
Sex," p. 28, also contend that "combative energy and sexual beauty
rise _pari passu_ with male katabolism."
[CT] "Darwinism," p. 293.
[CU] Mr. Poulton, who takes a similar line of argument in his "Colours
of Animals," lays special stress upon the production of _white_
(see p. 326).
[CV] See Chapter VIII.
[CW] "Darwinism," p. 172.
[CX] See "Animals and Plants under Domestication," vol. ii. p. 80.
[CY] "Darwinism," p. 332.
[CZ] "The Colour-Sense," by Grant Allen, p. 95.
[DA] That on "The Emotions of Animals" (X.).
[DB] "Darwinism," p. 318.
[DC] Natural History Society of Wisconsin, vol. i. (1889).
[DD] "Darwinism," p. 286.
[DE] On the negative character of disuse, see p. 196.
[DF] Cope, "Origin of the Fittest," p. 374.
[DG] It would appear, from certain passages of his "Darwinism," that
Mr. A. R. Wallace (e.g. p. 139, note) holds or held similar views.
"The genera _Ateles_ and _Colobus_," he says, "are two of the most
purely arboreal types of monkeys, and it is not difficult to
conceive that the constant use of the elongated fingers for
climbing from tree to tree, and catching on to branches while
making great leaps, might require all the nervous energy and
muscular growth to be directed to the fingers, the small thumb
remaining useless." I should also have quoted Mr. Wallace's
account of the twisting round of the eyes of flat-fishes--where he
says that the constant repetition of the effort of twisting the
eye towards the upper side of the head, when the bony structure is
still soft and flexible, causes the eye gradually to move round
the head till it comes to the upper side--had he not subsequently
disclaimed this explanation (see _Nature_, vol. xl. p. 619). It is
possible that Mr. Wallace, notwithstanding the words "constant
use" in the passage I have quoted, merely intends to imply that
the elongated fingers are of advantage in climbing, and are thus
subject to natural selection, the thumb diminishing through
economy of growth.
[DH] I find, on rereading one of his articles, that I have here
unwittingly adopted one of Mr. Romance's arguments (see _Nature_,
vol. xxxvi. p. 406). The instance Mr. Romanes cites is the curious
habit of dogs turning round before they lie down.
[DI] Mr. Darwin, while contending that the modifications need not all
have been simultaneous, says, "Although natural selection would
thus tend to give the male elk its present structure, yet it is
probable that the inherited effects of use, and of the mutual
action of part on part, have been equally or more important"
("Animals and Plants under Domestication," vol. ii. p. 328).
[DJ] _Midland Naturalist_, November, 1889.
[DK] See _ante_, p. 52.
[DL] _Nature_, vol. xli. p. 511.
[DM] "Animals and Plants under Domestication," vol. ii. p. 291.
[DN] In the third chapter we saw that in such cases not only are there
an enormous number of ova produced, but that (e.g. in aurelia and
the liver-fluke) each ovum produces, through the intervention of
asexual multiplication, many individuals.
[DO] Cope, "Origin of the Fittest," pp. 226, 125, and 297.
[DP] "Animals and Plants under Domestication," vol. ii. p. 313.
[DQ] Ibid. p. 56.
[DR] _Nature_, vol. xxxvi. p. 592.
[DS] Quoted from "Medical Notes and Reflections," 1855, p. 267, by
Darwin, "Animals and Plants under Domestication," vol. i. p. 446.
[DT] Darwin, "Animals and Plants under Domestication," vol. i. p. 465.
[DU] "Natural Inheritance," p. 12.
[DV] Darwin, "Animals and Plants under Domestication," vol. ii. p. 70.
[DW] "Organic Evolution," Mr. Cunningham's translation, p. 76.
[DX] Darwin, "Animals and Plants under Domestication," vol. i. p. 104.
[DY] Similarly, from a chance sport of a one-eared rabbit, Anderson
formed a breed which steadily produced one-eared rabbits ("Animals
and Plants under Domestication," vol. i. p. 456). This is an
example of asymmetrical variation. Variations are generally, but
not always, symmetrical. Superficial colour-variations are
sometimes asymmetrical. Gasteropod molluscs are nearly always
asymmetrically developed. Among insects, _Anisognathus_ affords an
example of the asymmetrical development of the mandible. Our
right-handedness is a mark of asymmetry.
[DZ] "Natural Inheritance," p. 32.
[EA] See "Animals and Plants under Domestication," vol. ii. p. 40, from
which illustrations are taken.
[EB] "Evolution and Disease," p. 169.
[EC] "Animals and Plants under Domestication," vol. ii. p. 8.
[ED] "Darwinism," p. 107.
[EE] Darwin, "Animals and Plants under Domestication," vol. ii. pp. 17,
18.
[EF] "Animals and Plants under Domestication," vol. ii. p. 201.
[EG] Ibid. p. 282. The phenomena of the seasonal dimorphism of
butterflies and moths show that changes of temperature (and
perhaps moisture, etc.) determine very striking differences in
these insects.
[EH] "Animals and Plants under Domestication," vol. ii. p. 244.
[EI] "Darwinism," p. 293.
[EJ] "Evolution of Sex," p. 22.
[EK] "Incidental Observations in Pedigree Moth-breeding," F.
Merrifield. Transactions Entomological Society, 1889, pt. i. p.
79, _et seq._
[EL] _Nature_, vol. xli. p. 393.
[EM] See Professor Meldola's edition of Professor Weismann's "Studies
in the Theory of Descent," and Mr. Cunningham's translation of
Professor Eimer's "Organic Evolution."
[EN] See Darwin, "Animals and Plants under Domestication," vol. ii. p.
252.