THE FACTORS OF EVOLUTION
Variation along definite lines and Natural Selection are undoubtedly
important factors of evolution—Whether or not sexual selection is a
factor we are not yet in a position to decide—_Modus operandi_ of
Natural Selection—Correlation an important factor—Examples of
correlation—Correlation is a subject that requires close
study—Isolation a factor in evolution—Discriminate
isolation—Indiscriminate isolation—Is the latter a factor?—Romanes’
views—Criticism of these—Indiscriminate isolation shown to be a
factor—Summary of the methods in which new species arise—Natural
Selection does not make species—It merely decides which of certain
ready-made forms shall survive—Natural Selection compared to a
competitive examination and to a medical board—We are yet in darkness
as to the fundamental causes of the Origin of Species—In experiment and
observation rather than speculation lies the hope of discovering the
nature of these causes.
We have so far considered three factors of evolution. The first of these
is the tendency of organisms to vary along definite lines. This is a most
important factor, because, unless variation occurs in any given
direction, there can be no evolution in that direction. Variations are
the materials upon which the other factors, or causes, of evolution work.
The second great factor is natural selection. Natural selection may be
compared to a builder, and variations to his materials. The kind of
building that a builder can construct depends very largely on the
material supplied to him. The Forth Bridge could not have been built had
those who constructed it had no material given them but bricks and
mortar. Wallaceians regard natural selection as a builder who is supplied
with every kind of building material—stone, bricks, wood, iron,
aluminium, in any quantities he may desire. They therefore regard natural
selection as the one and only cause which determines evolution. This,
however, is a wrong idea. Natural selection should rather be likened to a
builder who is supplied with a limited variety of building materials, so
that considerable restrictions are imposed on his building operations.
The doors, windows, fireplaces, etc., are supplied to him ready-made. He
merely selects which of these he will use for each building.
The third factor of evolution which we have considered is sexual
selection. As we have seen, sufficient attention has not been paid to
this subject, so that we are not yet in a position to say how much, if
any, influence it has exercised on the course of evolution.
The Struggle for Existence
In addition to these three factors, there are, we believe, some others.
Before proceeding to a consideration of these, it is important to study
carefully the _modus operandi_ of natural selection, or, in other words,
the nature of the struggle for existence, as many of the statements
contained in recent books on evolution seem to us to be based upon a
mistaken conception of this important factor.
As usual, Darwin’s disciples have failed to improve upon the account he
gave of the nature of the struggle for existence. This is set forth in
Chapter III. of the _Origin of Species_.
“The causes,” writes Darwin (new edition, p. 83), “which check the
natural tendency of each species to increase in number are most obscure.
Look at the most vigorous species; by as much as it swarms in numbers, by
so much will it tend to increase still further. We know not exactly what
the checks are even in a single instance.” This is perfectly true.
Nevertheless elaborate theories of protective and warning colouration and
mimicry have been built up on the tacit assumption that the checks to the
multiplication of all, or nearly all, species are the creatures which
prey upon them. Possibly no Wallaceian asserts this in so many words, but
it is a logical deduction from the excessive prominence each one gives to
the various theories of animal colouration; for, if the chief foes of an
organism are not the creatures which prey upon it, how can the particular
shade and pattern of its coat be of such paramount importance to it?
Checks on Increase
We shall endeavour to show that there are checks on the increase of a
species far more potent than the devastation caused by those creatures
which feed upon it. Let us, however, first briefly set forth some of the
checks on the multiplication of organisms which Darwin mentions in the
_Origin of Species_.
“Eggs, or very young animals,” he says, “seem generally to suffer the
most, but this is not invariably the case.” This is, as we have already
insisted, a most important point to be borne in mind, especially when
considering the various current theories of animal colouration. When once
the average animal has become adult its chances of survival are
enormously increased.
A second check mentioned by Darwin is the limitation of food supply. “The
amount of food for each species,” he writes (p. 84), “of course gives the
extreme limit to which each can increase; but very frequently it is not
the obtaining food, but the serving as prey to other animals, which
determines the average numbers of a species. Thus there seems to be
little doubt that the stock of partridges, grouse, and hares on any large
estate depends chiefly on the destruction of vermin. . . . On the other
hand, in some cases, as with the elephant and rhinoceros, none are
destroyed by beasts of prey.”
We are inclined to think that neither the food limit nor the beasts of
prey are a very important check on the multiplication of organisms. The
lion, for example, was never so numerous as to reach the limit of its
food supply. Before the white man obtained a foothold in Africa vast
herds of herbivores were to be seen in those districts where lions were
most plentiful. This is a most important fact, for, if the numbers of a
species are not determined by those of the animals that prey upon it, the
particular colour of an organism is probably not of any direct importance
to it. This cuts away the foundation of some of the generally accepted
theories of animal colouration.
“Climate,” writes Darwin (p. 84), “plays an important part in determining
the average numbers of a species, and periodical seasons of extreme cold
or drought seem to be the most effective of all checks. I estimated
(chiefly from the greatly reduced numbers of nests in the spring) that
the winter of 1854-55 destroyed four-fifths of the birds in my own
grounds, and this is a tremendous destruction when we remember that 10
per cent. is an extraordinarily severe mortality from epidemics with
man.”
In our opinion, Darwin did not lay nearly enough stress upon the
importance of climate as a check on the increase of species. We have seen
that he stated his belief that it is the most effective of all checks.
But even this is not a sufficiently strong statement of the case. It
seems to us that before this check all other checks pale into
insignificance.
Darwin failed to notice the potent effects of damp. Damp is more
injurious to most species than even cold or drought, as every one who has
tried to keep birds in England knows. All entomologists are aware how
harmful damp is to insects. Caterpillars seem to take cover under leaves
to avoid damp rather than to hide themselves from birds, since these make
a point, when searching for insects, of invariably looking carefully
under leaves.
It is a well-known fact that a wet winter in England causes much
mortality among rabbits. The increase of the rabbit in Australia is
usually attributed to the fact that the little rodent has not so many
predatory creatures to contend with there as it has in Europe. This is
not so. In Australia the rabbit has to fight against eagles, other large
birds of prey, carnivorous marsupials, feral cats, monitor lizards and
large snakes, to say nothing of the well-organised and persistent attacks
of man.
Were predacious creatures the most important foes of the rabbit it would
never have obtained a firm foothold in Australia. Damp appears to be its
chief enemy. In Australia this does not exist. Hence the remarkable
increase of the species. Stronger evidence it would not be possible to
advance of the potency of damp as a check on the increase of a species
and of the comparative powerlessness of the attacks of raptorial
creatures.
The failure of the sandgrouse to establish a footing in England is, we
believe, due to the fact that it is constitutionally unfitted to
withstand our damp climate.
The camel is an animal that revels in dry habitats, hence the difficulty
of keeping camels in damp Bengal, although they seem to thrive well
enough in the drier parts of India.
“When a species,” writes Darwin (p. 86), “owing to highly favourable
circumstances, increases inordinately in numbers in a small tract,
epidemics—at least, this seems generally to occur with our game
animals—often ensue; and here we have a limiting check independent of the
struggle for life. But even some of these so-called epidemics appear to
be due to parasitic worms, which have from some cause, possibly in part
through facility of diffusion amongst the crowded animals, been
disproportionately favoured: and here comes in a sort of struggle between
the parasite and its prey.”
Thus inadequately does Darwin deal with that bar to the increase of
organisms, which is only second in importance to the effect of climate.
The check occasioned by disease and parasites is one to which naturalists
have as yet paid but little attention. The result is a very general
misunderstanding of the true nature of the struggle for existence, in
other words, of the _modus operandi_ of natural selection.
The tsetse-fly in Africa is a far more important check on the increase of
some animals than the lions and other beasts of prey. There are in that
continent large tracts of country, known as tsetse-fly belts, in which
neither horse, nor ox, nor dog can exist. If races of these animals were
to arise which could withstand the bite of the tsetse-fly, these species
might increase more rapidly than the rabbit in Australia has done, nor
would it matter if the creatures in question were bright crimson, or any
other conspicuous colour.
Take the case of the lion in Africa. The chief bar to the increase in
numbers of this species appears to be the teething troubles to which the
whelps are liable. Now suppose that a mutation were to occur in the lion.
Suppose that several members of a litter were all bright blue, and that
these suffered from no teething troubles. They would probably all grow
up, and although at some disadvantage as hunters on account of their
conspicuous colouring, they would nevertheless probably increase at the
expense of the normally coloured lions, because of the immunity of their
offspring from death from teething troubles. Zoologists would then be at
a loss to explain their bright colouring. We should have all manner of
ingenious suggestions raised, namely, that in the moonlight these
creatures were really not at all conspicuous, indeed that they were
obliteratively coloured. In other words, a totally wrong explanation of
their colouring would be given and accepted. It is our belief that many
of the explanations put forward and accepted of the colouration of
existing species are wide of the mark.
As all bee-keepers are aware, the disease known as foul-brood works more
havoc among their bees than all the insectivorous creatures put together.
Similarly throat disease among wood-pigeons does more towards keeping
their numbers down than all the efforts of predacious birds.
A check on multiplication not mentioned by Darwin is that which is
sometimes imposed by the individuals of the species on one another. Thus,
in some animals, as, for example, the hyæna, the male occasionally
devours his own young ones.
A check of a similar nature results from the habit which the Indian House
Crow (_Corvus splendens_) has of interrupting the pairing operations of
its neighbours.
Attributes of Successful Species
We are now in a position to sum up briefly the more important requisites
for success in the struggle for existence.
These are not so much specialised structure as courage, a good
constitution, mental capacity and prolificacy.
Few animals possess all these characteristics in a pre-eminent degree,
for, to use the words of Mr Thompson Seton, “Every animal has some strong
point or it could not live, and some weak point or the other animals
could not live.” Courage may be of two kinds—active courage, like that of
the Englishman, or passive courage, like that of the Jew.
As D. Dewar has said: In the struggle for existence, “An ounce of good
solid pugnacity is worth many pounds of protective colouration.”
It is of course possible for an animal to possess too much courage. An
excessive amount of courage will often cause a creature to fight
unnecessary battles, which may lead to its premature death. This is
perhaps the reason why the pugnacious black form of the leopard is not
more numerous.
Under a good constitution we must include the power of resisting the
rigours of climate, more especially damp, the ability to resist disease,
and the enjoyment of a good digestion. When from any cause the normal
food of a species becomes scarce, the members of that species will have
to starve or supplement the normal diet with food of an unusual nature;
and those that are endowed with a good digestion will be able to digest
the new food and thus survive, while those which cannot assimilate food
to which they are unaccustomed will become emaciated and perish. We see
this in every hard winter in England, when the redwing, which, unlike
other thrushes, cannot thrive on berries, is the first to die. Most of
the more successful birds—the crows and gulls, for example—are
omnivorous—that is to say, they are able to digest all manner of food.
Under mental capacity, we would include cunning and sufficient
intelligence to adapt oneself to changed conditions. It is largely
through man’s superior mental capacity that he has become the dominant
species. It is true that he displays also courage and a good
constitution, being able to adapt himself to life under the most diverse
conditions; but this is, of course, in part due to his mental capacity,
which enables him to some extent to adapt his environment to himself.
The advantages of prolificacy are so apparent that it is unnecessary to
dilate upon them. Nearly as important as excessive fertility is the
ability on the part of the parents to look after their young ones.
Every successful species possesses in a special degree at least one of
the above attributes. It is interesting to take in turn the various
species which are most widely distributed and consider to what extent
they possess these several qualities.
Let us now consider a factor in evolution which is nearly as important as
natural selection itself—we allude to the phenomenon of correlation.
Correlation
We may define correlation as the interdependence of two or more
characters. This phenomenon is far more common than the majority of
naturalists seem to think. It very frequently happens that one particular
character never appears in an organism without being accompanied by some
other character which we should not expect to be in any way related to
it.
Darwin called attention to this phenomenon. “In monstrosities,” he
writes, on page 13 of the _Origin of Species_ (new edition), “the
correlations between quite different parts are very curious, and many
interesting instances are given in Isidore Geoffroy St Hilaire’s great
work on this subject. Breeders believe that long limbs are almost always
accompanied by an elongated head. Some instances of correlation are quite
whimsical: thus cats which are entirely white and have blue eyes are
generally deaf; but it has been lately stated by Mr Tait that this is
confined to the males.
“Colour and constitutional peculiarities go together, of which many
remarkable cases could be given among animals and plants. From the facts
collected by Heusinger, it appears that white sheep and pigs are injured
by certain plants, whilst dark-coloured individuals escape. Professor
Wyman has recently communicated to me a good illustration of this fact:
on asking some farmers in Virginia how it was that all their pigs were
black, they informed him that the pigs ate the paint-root
(_Lachnanthes_), which coloured their bones pink, and which caused the
hoofs of all but the black varieties to drop off; and one of the
‘crackers’ (_i.e._ Virginia squatters) added, “we select the black
members of a litter for raising, as they alone have a good chance of
living.’
“Hairless dogs have imperfect teeth; long-haired and coarse-haired
animals are apt to have, as is asserted, long or many horns; pigeons with
feathered feet have skin between their outer toes; pigeons with short
beaks have small feet, and those with long beaks large feet.
“Hence, if man goes on selecting, and thus augmenting, any peculiarity,
he will almost certainly modify unintentionally other parts of the
structure, owing to the mysterious laws of the correlation of growth.”
The great importance of the principle of the correlation of organs is,
that _natural selection may indirectly cause the survival of unfavourable
variations, or of variations which are of no utility to the organism,
because they happen to_ _be correlated with organs or structures that are
useful_.
Physiologists insist more and more upon the close interdependence of the
various parts of the organism. All recent researches tend to show that
each of the organs has, besides its primary function, a number of
subordinate duties to perform, and that the removal of one organ reacts
on all the others.
In face of these facts we should have expected those zoologists who have
followed Darwin to have paid very close attention to the subject of
correlation. As a matter of fact, the phenomenon seems to have been
almost completely neglected. This is an example of the manner in which
the superficial theories which to-day command wide acceptance have tended
to bar the way to research.
There seems to be, in the case of some organisms, at any rate, a distinct
correlation between their colouring and their constitution or mental
characters. For example, the black forms of the cobra, the leopard, and
the jaguar are notoriously bad-tempered.
“There is,” writes Col. Cunningham, on p. 344 of _Some Indian Friends and
Acquaintances_, “much variation in the temper of different varieties of
cobras, and, as is often so noticeable among other sorts of animals,
there would seem to be a distinct correlation between darkness of colour
and badness of temper. It is probably in part owing to a recognition of
this that the cobras ordinarily seen in the hands of the so-called snake
charmers are of a very light colour, although the choice may also be to
some extent of æsthetic origin, seeing that the paler varieties are
specially ornamental, due to the brilliancy of their markings and the
great development of their hoods.” It would thus appear that there is
also a correlation between the colour of the cobra and the size of its
hood.
Hesketh Pritchard informs us, in _Through the Heart of Patagonia_, that
the Gauchos assert that a “picaso” colt—that is to say, a black one with
white points—is the reverse of docile. Similarly, black mice are said to
be very hard to tame.
We have already called attention to the importance of courage and the
power of resisting the rigours of climate in the struggle for existence.
It is apparently because black is so frequently correlated with courage
that it is seen comparatively often in nature, in spite of the fact that
it is a very bad colour as regards protection from enemies. Those birds
and beasts which are black are usually thriving species. The dominance of
the crow tribe is a case in point. Crows, it is true, are not really
courageous, but they are dangerous owing to their gregarious habits, and
are dreaded by other creatures on account of their power of combination.
In _Birds of the_ _Plains_, D. Dewar records an instance of a number of
crows killing in revenge so powerful a bird as the kite.
Since very many species seem to throw off melanistic variations, it may
perhaps be asked, How is it that more black species do not exist?
The reply is twofold. In the first place, it is quite likely that in some
organisms black variations are not correlated with courage or extreme
pugnacity, and when such is the case the melanistic varieties will be
more likely to be exterminated by foes, on account of their
conspicuousness. It must be remembered that, other things being equal,
the inconspicuously coloured organism has a better chance of survival
than the showily coloured one. This is, of course, a very different
attitude from that which insists on the all-importance to animals of
protective colouration. Secondly, it is not difficult to see how too much
courage may be fatal to an animal in leading it to take risks which a
more timid creature would refrain from doing. This, as we have already
suggested, is probably the reason why the black panther is so scarce. The
black colour is readily inherited, so there must be some cause which
tends to kill off the black varieties of the panther.
Lest it be thought the idea that excessive courage and pugnacity are
harmful is mere fancy, let us quote from the account of the nesting
habits of the White-rumped Swallow (_Tachycineta leucorrhoa_) given by Mr
W. H. Hudson on p. 32 of _Argentine Ornithology_. He says that no matter
how many nesting sites are available, there is always much fighting
amongst these birds for the best places. “Most vindictively,” he writes,
“do the little things clutch each other, and fall to the earth twenty
times an hour, where they often remain struggling for a long time,
heedless of the screams of alarm their fellows set up above them; for
often, while they thus lie on the ground punishing each other, they fall
an easy prey to some wily pussy who has made herself acquainted with
their habits.”
We have already emphasised the importance to many species of possessing
the power of resisting the effects of damp. In the case of some organisms
favourable variations in this direction may possess a greater survival
value than those in the shape of greater speed or physical strength.
Now, if there be any correlation between the power of resisting damp and
the colour an animal bears, it is quite probable that animals of this
colour, whether or no it be conspicuous, are likely to survive in
preference to those who are more protectively coloured. There is some
evidence that in certain cases, at any rate, resistance to climate is
correlated with colour peculiarities. For example, some fanciers assert
that yellow-legged poultry resist cold and damp better than those whose
legs are not yellow. Fowls which have yellow legs have also yellow skins.
In this connection the almost universal assumption of orange feet by
domestic guinea-fowls is significant. Normally the feet of these birds
are black, and their natural African habitat is a dry one.
A grey or white colour appears to be correlated with resistance to cold.
In birds this may perhaps be explained by the fact that the feathers in
some light-coloured varieties are longer than in those of
normally-coloured ones. Thus mealy-coloured canaries have longer feathers
than brightly-coloured ones.
The Arctic Skua, having no enemies to fear, stands in no need of
protective colouration. It would therefore seem that the white-breasted
form of this bird becomes more numerous as it nears the north pole, not
because of the closer assimilation of its plumage to the colour of the
snowy surroundings, but because the bird has to resist the greater degree
of cold the farther north it finds itself. Similarly, in the region of
the south pole the albino form of the Giant Petrel (_Ossifraga gigantea_)
becomes common. Both these birds are themselves predatory and not liable
to be preyed upon.
The curious china-white legs of some desert birds—as, for example,
coursers and larks—would seem to indicate a power of resisting the hot
rays radiating from the sand on which these creatures dwell.
White quills do not wear well either in domestic birds or in wild
albinos. This may explain why it is that when a white wild species of
bird has any black in its plumage the black is almost invariably on the
tips of the wings.
White quill-feathers are one of the commonest variations observed in
domesticated birds, nevertheless they are as rare as complete whiteness
among birds in their natural state.
A chestnut or bay colour in mammals appears to be correlated with a high
rate of speed, as in the thoroughbred horse. This perhaps explains why so
many of the swiftest species of antelope, such as the hartebeests and
sassaby (_Damaliscus lunatus_), are chestnut bay in colour. It is further
a remarkable fact that in the Black-buck (_Antilope cervicapra_) and the
Nilgai (_Boselaphus tragocamelus_) the females, which are faster than the
males, are not black or grey like their respective males, but reddish.
Wild turkeys are bronze; tame ones are black more often than any other
colour. This may be due to the fact that in them nigritude is correlated
with the power to resist damp. Among human beings those races which live
in very swampy districts are often intensely black.
It is a significant fact that those domestic animals which are bred for
speed or for fighting purposes do not assume all the varied hues that
characterise those that are allowed to breed indiscriminately.
Racehorses, greyhounds, and homing pigeons furnish examples of this. Even
more remarkable is the case of the Indian Aseel or game-cock. This is
bred purely for fighting purposes, and is required to display
extraordinary powers of endurance, since the spurs are cut off in order
to prolong the fight. Thus it is that this Indian race of game-cocks
shows little variation when compared with the English breed, which fights
in a more natural manner. The hens of the Indian form seem never to show
the colouration of the wild jungle fowl, although the cocks may do so. It
would appear that hens having the colouration of their wild ancestors
cannot breed cocks possessed of the requisite courage. The Aseel is said
to be of the highest courage only when the legs, beak and iris are white.
There is, we believe, not the least doubt that many other connections
between colour and various characteristics have yet to be discovered. It
is high time that competent naturalists paid attention to this subject. A
study of the question will almost certainly throw much light upon many
phenomena of animal colouration which hitherto have not been
satisfactorily explained. It is quite likely that the sandy hue displayed
by birds and beasts which frequent desert regions may be due to a
correlation with the power of withstanding intense dry heat rather than
to its rendering them inconspicuous to their foes.
As other examples of correlation we may cite the correlation which seems
to obtain between short canine teeth and the absence of a hairy covering
to the body. This phenomenon is observed both in men and pigs. Hairless
dogs almost invariably have their teeth but poorly developed.
Darwin called attention to the connection between a short beak and small
feet in pigeons; we see the same phenomenon in the dwarf breed of ducks
known as call-ducks.
A curious correlation exists between fowls’ eggs with brown shells and
the incubating habit. Fanciers have long tried in vain to produce a hen
that lays brown eggs without becoming “broody” at certain seasons.
Among fowls, long legs are invariably correlated with a short tail, as is
well seen in the Malay breed. This correlation may explain the short
tails of wading birds. Short-legged fowls, like Japanese bantams, have
long tails, and it is significant that the short-legged Weka Rails
(_Ocydromus_) of New Zealand have unusually long tails for the family. In
this connection we may say that the tail-like plumes of the cranes are
not tail-feathers, but the tertiary feathers of the wings. As egrets also
have long trains of plumes growing from the back, it cannot be said that
the short tail of the vast majority of the waders is due to the fact that
these birds would be at a disadvantage were their caudal feathers long.
Isolation
Isolation is a most important factor in the making of species. It is a
factor to which Darwin failed to attach sufficient importance, and one
which has been to a large extent neglected by Wallaceians.
Divergence of Character
We have seen how a species can be improved or changed by natural
selection. All those individuals which have varied in a favourable
direction have been preserved, and allowed to leave behind them offspring
that inherit their peculiarities, while those which have not so varied
have perished without leaving behind any descendants. Thus the nature of
the species has changed. The old type has given place to a new one.
Instead of species A, species B exists. This is what Romanes has called
_monotypic_ evolution—the transformation of one species into another
species. But any theory of the origin of species must be able to answer
the question, Why have species multiplied? How is it that species A has
given rise to species B, C, and D, or, while itself continuing to exist,
has thrown off sister species B and C? How is it that in the course of
evolution, species have not been transmuted in linear series instead of
ramifying into branches? This ramification of a species into branches has
been termed by Romanes _polytypic_ evolution. It is easy to see how
natural selection can bring about monotypic evolution, but how can it
have effected polytypic evolution? To use Darwin’s phraseology, how is it
that divergence of character has come about? Darwin’s reply to this
question is (_Origin of Species_, p. 136), “from the simple circumstance
that the more diversified the descendants from any one species become in
structure, constitution, and habits, by so much will they be better
enabled to seize on many and widely diversified places in the polity of
nature, and so be enabled to increase in numbers.
“We can clearly discern this in the case of animals with simple habits.
Take the case of a carnivorous quadruped, of which the number that can be
supported in any country has long ago arrived at its full average. If its
natural power of increase be allowed to act, it can succeed in increasing
(the country not undergoing any change in its conditions) only by its
varying descendants seizing on places at present occupied by other
animals: some of them, for instance, being enabled to feed on new kinds
of prey, either dead or alive; some inhabiting new stations, climbing
trees, frequenting water, and some perhaps becoming less carnivorous. The
more diversified in habits and structure the descendants of our
carnivorous animal become, the more places they will be enabled to
occupy. What applies to one animal will apply throughout all time to all
animals—that is, if they vary—for otherwise natural selection can effect
nothing.” Darwin was, therefore, of opinion that natural selection is
able to bring about polytypic evolution. Darwin tacitly assumes, in the
illustration he gives, that the various races of the carnivorous animal
are in some way prevented from intercrossing; for if they interbreed
indiscriminately, these races will tend to be obliterated.
Isolation
“That perfectly free intercrossing,” writes Professor Lloyd Morgan (on p.
98 of _Animal Life and Intelligence_), “between any or all of the
individuals of a given group of animals is, so long as the characters of
the parents are blended in the offspring, fatal to divergence of
character, is undeniable. Through the elimination of less favourable
variations, the swiftness, strength, and cunning of a race may be
gradually improved. But no form of elimination can possibly differentiate
the group into swift, strong, and cunning varieties, distinct from each
other, so long as all three varieties freely interbreed, and the
characters of the parents blend with the offspring. Elimination may and
does give rise to progress in any given group, _as a group_; it does not
and cannot give rise to differentiation and divergence, so long as
interbreeding with consequent interblending of characters be freely
permitted. Whence it inevitably follows, as a matter of simple logic,
that where divergence has occurred, intercrossing and interbreeding must
in some way have been lessened or prevented.
“Thus a new factor is introduced, that of _isolation_ or _segregation_.
And there is no questioning the fact that it is of great importance. Its
importance, indeed, can only be denied by denying the swamping effects of
intercrossing, and such denial implies the tacit assumption that
interbreeding and interblending are held in check by some form of
segregation. The isolation explicitly denied is implicitly assumed.”
This is very sound criticism, and is not very materially affected by the
fact that the intercrossing of varieties does not necessarily imply a
blending of their characters in the offspring; for, as we have seen, some
characters do not blend. No matter what form inheritance takes, in order
that natural selection may cause polytypic evolution it must be assisted
by isolation in some form or other.
Thus isolation is an important factor in evolution, though probably not
so important as its more extreme advocates would have us believe. Wagner,
Romanes, and Gulick have, in insisting upon the importance of the
principle of isolation, rendered valuable service to biological science,
but, in common with most men having a new theory, they have pushed their
conclusions to absurd lengths.
As Romanes has pointed out, isolation may be discriminate or
indiscriminate. “If,” he writes, on p. 5 of vol. iii. of _Darwin and
after Darwin_, “a shepherd divides a flock of sheep without regard to
their characters, he is isolating one section from the other
indiscriminately; but if he places all the white sheep in one field, and
all the black sheep in another field, he is isolating one section from
the other discriminately. Or, if geological subsidence divides a species
into two parts, the isolation will be indiscriminate; but if the
separation be due to one of the sections developing, for example, a
change of instinct determining migration to another area, or occupation
of a different habitat on the same area, then the isolation will be
discriminate, so far as the resemblance of instinct is concerned.”
Discriminate Isolation
Other names for indiscriminate isolation are separate breeding and
apogamy. Discriminate isolation is also called segregate breeding and
homogamy. The human breeder resorts to discriminate isolation in that he
separates all those creatures from which he seeks to breed, from those
from which he does not wish to breed. Natural selection itself is,
therefore, a kind of discriminate isolator, since it isolates the fit by
destroying all the unfit, and, inasmuch as it kills off all those
creatures which it fails to isolate, it differs from other forms of
isolation in preventing the inter-breeding of the unisolated forms and
their giving rise to a different race. Thus it is clear that natural
selection, unless aided by some other form of isolation, can give effect
to only monotypic evolution. This is a point on which Romanes rightly
insists strongly.
There are several other forms of discriminate isolation. Sexual selection
would be one of these. Suppose, for example, that in any species there
are large and small varieties formed, and like tends to breed with like,
then the small individuals will breed with other small individuals, while
large ones will mate with large ones; thus two races—a large one and a
small one—will be evolved side by side, provided, of course, natural
selection does not step in and destroy one of them.
Another kind of discriminate isolation may be due to the fact that one
variety is ready to pair before the other; thus two races are likely to
arise which breed at different seasons. It is unnecessary for us to
discourse further on the subject of discriminate isolation; those
interested in the subject should read vol. iii. of _Darwin and after
Darwin_, by Romanes.
Indiscriminate Isolation
It is impossible to deny the importance of discriminate isolation as a
factor in evolution. On this there can be no room for disagreement among
biologists. It is when we come to the subject of indiscriminate isolation
that we enter a region of zoological strife.
Is indiscriminate isolation _per se_ a factor of evolution? Romanes,
Gulick, and Wagner assert that it is, Wallace and his adherents assert
that it is not.
As the burden of proof is on the former, they are entitled to the first
hearing.
“We may well be disposed, at first sight,” writes Romanes (_Darwin and
after Darwin_, p. 10), “to conclude that this kind of isolation can count
for nothing in the process of evolution. For if the fundamental
importance of isolation in the production of organic forms be due to its
segregation of like with like, does it not follow that any form of
isolation which is indiscriminate must fail to supply the very condition
on which all the forms of discriminate isolation depend for their
efficacy in the causing of organic evolution? Or, to return to one’s
concrete example, is it not self-evident that the farmer who separated
his flock into two or more parts indiscriminately, would not effect any
more change in his stock than if he had left them all to breed together?
Well, although at first sight this seems self-evident, it is, in fact,
untrue. For, unless the individuals which are indiscriminately isolated
happen to be a very large number, sooner or later their progeny will come
to differ from that of the parent type, or unisolated portion of the
parent stock. And, of course, as soon as this change of type begins, the
isolation ceases to be indiscriminate; the previous apogamy has been
converted into homogamy, with the usual result of causing a divergence of
type. The reason why progeny of an indiscriminately isolated section of
an originally uniform stock—_e.g._ of a species—will eventually deviate
from the original type is, to quote Mr Gulick, as follows:—‘No two
portions of a species possess exactly the same average character, and the
initial differences are for ever reacting on the environment and on each
other, in such a way as to ensure increasing divergence as long as the
individuals of the two groups are kept from intergenerating.’”
The words of Mr Gulick require close scrutiny. We may admit that “no two
portions of a species possess exactly the same average character,” but
why should the two, if prevented from interbreeding yet subjected to
similar climatic and other conditions, present the phenomenon of
“increasing divergence?” The reason assigned by Romanes is the “Law” of
Delbœuf, which runs:—“_A constant cause of variation_, however
insignificant it may be, changes the uniformity of type little by little,
and diversifies it _ad infinitum_.” From this “Law” it follows, says
Romanes, on p. 13 of vol. iii. _Darwin and after Darwin_, that “no matter
how infinitesimally small the difference may be between the average
qualities of an isolated section of a species compared with the average
qualities of the rest of that species, if the isolation continues
sufficiently long, differentiation of specific type is necessarily bound
to ensue.”
This deduction involves two important assumptions. The first is, that in
each of the separated portions of the given species there is a constant
cause of variation operating in one direction in the case of one portion
and in another direction in the case of the other. This assumption is,
unfortunately, not founded on fact. If we were to take one hundred
race-horses and shut them up in one park and one hundred cart-horses and
shut them up in another park, and prevent the interbreeding of the two
stocks, we should, if Romanes’s tacit assumption be true, see the two
types diverge more and more from one another. We know that as a matter of
fact they will tend, generation after generation, to become more like one
another. Galton’s Law of Regression, of which we have already spoken, and
which is supported by ample evidence, clearly negatives this tacit
assumption made by Romanes and Gulick. The second assumption upon which
their reasoning is based is that there is no limit to the amount of
change which can be effected by the accumulation of fluctuating
variations; but, as we have already seen (on p. 70), there is a very
definite limit and this limit is quickly reached.
Thus the arguments of Romanes and Gulick are fundamentally unsound.
Mollusca of Sandwich Isles
But the fact remains, and has to be accounted for, that, as a general
rule, when two portions of a species are separated, so that they are
prevented from interbreeding, they begin to diverge in character, and the
longer they remain thus separated the greater becomes that divergence.
This is an observed fact which cannot be gainsaid.
It was the observance of this fact which led Gulick to insist with such
emphasis on the importance of geographical isolation as a factor in
evolution. He discovered that the land mollusca of the Sandwich Islands
fall into a great number of varieties.
These islands are very hilly, and Gulick found that each of the varieties
is confined not merely to one island, but to one valley. “Moreover,”
writes Romanes, on p. 16 of _Darwin and after Darwin_, “on tracing this
fauna from valley to valley, it is apparent that a slight variation in
the occupants of valley 2, as compared with those of the adjacent valley
1, becomes more pronounced in the next, valley 3, still more so in 4,
etc., etc. Thus it was possible, as Mr Gulick says, roughly to estimate
the amount of divergence between the occupants of any two given valleys
by measuring the number of miles between them. . . . The variations which
affect scores of species, and themselves eventually run into fully
specific distinctions, are all more or less finely graduated as they pass
from one isolated region to the next; and they have reference to changes
of form or colour, which in no one case presents any appearance of
utility.”
Hitherto three different attempts have been made to explain this and
allied phenomena:—
1. That it is the result of isolation.
2. That it is the result of natural selection.
3. That it is the result of the action of the environment on the
organism.
Let us consider these in inverse order.
Local Species
In the case of some organisms, more especially plants, invertebrates, and
fish, the environment does exert a direct influence on their colouration.
But, as we have seen, the changes in colour, etc., thus induced appear
never to be transmitted to the offspring of the organisms so affected.
They disappear when the offspring are removed to other surroundings.
On the other hand, local races or species—as, for example, the
white-cheeked variety of sparrow found in India—usually retain their
external appearance when the environment is changed. In the one case the
peculiarity is not inherited; in the other it is inherited.
The Wallaceian explanation is, of course, that the phenomenon is the
result of natural selection. There must, say Wallace and his followers,
be some differences in the environment, differences which we poor human
beings cannot perceive, that have caused the divergence between the
various isolated sections of the species. In the case of some local
species this explanation is probably the correct one, but we have no
hesitation in saying that natural selection is unable to offer a
satisfactory explanation in a considerable number of instances. Take, for
example, the case of the land mollusca of the Sandwich Islands. Mr Gulick
worked for fifteen years at them, and states that so far as he is able to
ascertain the environment in the fifteen valleys is essentially the same.
“To argue,” writes Romanes, on p. 17 of vol. iii. of _Darwin and after
Darwin_, “that every one of some twenty contiguous valleys in the area of
the same small island must necessarily present such differences of
environment that all the shells in each are differently modified thereby,
while in no one out of the hundreds of cases of modification in minute
respects of form and colour can any human being suggest an adaptive
reason therefore—to argue thus is merely to affirm an intrinsically
improbable dogma in the presence of a great and consistent array of
opposing facts.”
Men of science not infrequently charge the clergy with adhering to dogma
in face of opposing facts; it seems to us that many of the apostles of
science are in this respect worse offenders than the most orthodox of
Churchmen.
The example of the mollusca of the Sandwich Islands is by no means a
solitary one. D. Dewar cited some interesting cases in a paper recently
read before the Royal Society of Arts (p. 103 of vol. lvii. of the
Society’s Journal):
“The Indian robins present even greater difficulties to those who profess
to pin their faith to the all-sufficiency of natural selection. Robins
are found in nearly all parts of India, and fall into two species, the
brown-backed (_Thamnobia cambaiensis_) and the black-backed Indian Robin
(_Thamnobia fulicata_). The former occurs only in Northern India, and the
latter is confined to the southern portion of the peninsula. The hen of
each species is a sandy brown bird with a patch of brick-red feathers
under the tail, so that we cannot tell by merely looking at a hen to
which of the two species she belongs. The cock of the South Indian form
is, in winter, a glossy black bird, with a white bar in the wing, and the
characteristic red patch under the tail. The cock of the northern
species, as his name implies, has a sandy-brown back, which contrasts
strongly with the glossy black of his head, neck, and under parts. In
summer the cocks of the two species grow more like one another owing to
the wearing away of the outer edges of their feathers; but it is always
possible to distinguish between them at a glance. The two species meet at
about the latitude of Bombay. Oates states that in a certain zone, from
Ahmednagar to the mouth of the Godaveri valley, both species occur, and
they do not appear to interbreed.
“It seems impossible to maintain that natural selection, acting on minute
variations, has brought about the divergence between these two species.
Even if it be asserted that the difference in the colour of the feathers
of the back of the two cocks is in some way correlated with adaptability
to their particular environment, how are we to explain the fact that in a
certain zone both species flourish?
“A similar phenomenon is furnished by the red-vented bulbul. This genus
falls into several species, each corresponding to a definite locality and
differing only in details from the allied species, as, for example, the
distance down the neck to which the black of the head extends. There is a
Punjab Red-vented Bulbul (_Molpastes intermedius_), a Bengal (_Molpastes
bengalensis_), a Burmese (_Molpastes burmanicus_) and a Madras
(_Molpastes hæmorrhous_) species.
“It does not seem possible to maintain the contention that these various
species are the products of natural selection, for that would mean if the
black of the head of the Punjab species extended further into the neck
the bird could not live in that country.”
Thus, natural selection clearly is unable to explain some cases of
divergence of character due to geographical isolation.
There remains the third explanation, that the divergence is the result of
the simple fact of isolation.
We have already shown how insuperable are the objections to the view held
by Romanes and Gulick.
It seems to us that explanation must lie in the fact that mutations occur
every now and again in some species. If two portions of a species are
separated and a mutation occurs in one portion and not in the other, and
if the mutating form succeeds in supplanting the parent form in that
isolated portion of the species in which it has appeared, we should have
the phenomenon of two races or species differing in appearance although
subjected to what appear to be identical environment.
This, of course, is pure conjecture. All that can be said of it at
present is that it is not opposed to observed facts. That mutations do
occur must be admitted. At present we are totally in the dark as to what
causes them. They arise at the most unexpected times.
In favour of the explanation based on “mutation” there is the interesting
fact that geographical isolation does not by any means always cause
divergence of character. This Romanes, with great fairness, freely
admits. “There are,” he writes, on p. 133 of vol. iii. of _Darwin and
after Darwin_, “four species of butterflies, belonging to three genera
(_Lycæna donzelii_, _L. pheretes_, _Argynnis pales_, _Erebia manto_),
which are identical in the polar regions and the Alps, notwithstanding
that the sparse Alpine populations have been presumably separated from
their parent stocks since the glacial period.” Again, there are “certain
species of fresh-water crustaceans (_Apus_), the representatives of which
are compelled habitually to form small isolated colonies in widely
separated ponds, and nevertheless exhibit no divergence of character,
although apogamy has probably lasted for centuries.”
Cormorants
To these examples we may add that of the cormorants. These birds have an
almost worldwide range. One species—our Cormorant (_Phalacrocorax
carbo_)—occurs in every imaginable kind of environment. Isolation has not
effected any changes in the appearance of this species. Yet in New
Zealand there exist no fewer than fourteen other species of cormorant.
New Zealand is a country where climatic conditions are comparatively
uniform, nevertheless it boasts of no fewer than fifteen out of the
thirty-seven known species of cormorant. A possible explanation of this
phenomenon may be found in the comparatively easy conditions under which
cormorants live in New Zealand.[10] Under such circumstances mutants may
be permitted by natural selection to survive, whereas in other parts of
the world such mutants have not been able to hold their own.
Prof. Bateson has likened natural selection to a competitive examination
to which every organism must submit. The penalty for failure is immediate
death. The standard of the examination may vary with the locality.
Isolation, then, is a very important factor in the making of species, for
without it, in some form, the multiplication of species is impossible.
Let us, in conclusion, briefly summarise what we now know of the method
in which new species are made. We have studied the various factors of
evolution—variation and correlation, heredity, natural selection, sexual
selection, and the other kinds of isolation. How do these combine to
bring new species into being, and to establish the same?
Natural Selection
Let us first consider the factor known as natural selection, since this
is the one on which Darwin laid such great stress. Natural selection,
although a most important factor in evolution, is not an indispensable
one. Evolution is possible without natural selection.
Let us suppose that there is no such thing as natural selection; that the
numbers of existing species are kept constant by the elimination of all
individuals born in excess of the number required to maintain the species
at the existing figure, and that the elimination of the surplus is
effected, not by natural selection, but by chance, by the drawing of
lots. Under such circumstances there may be evolution, existing species
may undergo change, but the evolution will be determined solely by the
lines along which variations occur.
If mutations take place along certain fixed lines, and tend to accumulate
in the given directions, evolution will proceed along these lines quite
independently of the utility to the organism of the mutations that occur.
An unfavourable mutation will have precisely the same chance of survival
as a favourable one.
If, on the other hand, mutations occur indiscriminately on all sides of
the mean, then those mutations which happen to occur most frequently will
have the best chance of survival, and they will mark the lines of
evolution. But suppose that no mutation occurs more frequently than the
others. Under such circumstances there will be no evolution, unless, by
some cause or other, portions of the species are isolated, because in the
long run the mutations will neutralise one another.
Let us now suppose that natural selection comes into play. The old method
of determining by lot which forms shall persist is replaced by selection
on the fixed principle that the fittest shall survive. The mutations
appear as before, and as before, of the large number that occur, only a
few are permitted to survive. But now the survivors, instead of being a
motley crowd, are a selected band, composed of individuals having many
characteristics in common—a homogeneous company. Thus one result of
natural selection is to accelerate evolution, by weeding out certain
classes of individuals and preventing them breeding with those it has
selected. On the other hand, natural selection will tend to diminish the
number of species which have arisen through mutation, inasmuch as it
weeds out many mutants which would have perished had their survival been
determined by lot.
Origin of the Fittest
From this the kind of work performed by natural selection should be
obvious. Natural selection does not make new species. These make
themselves, or, rather, originate in accordance with the laws of
variation.
“You can,” runs an old proverb, “bring a horse to the drinking fountain,
but you cannot make him drink.” You may be able to bring a child into the
world, but you cannot secure its survival. Variation brings into being
mutants, which are incipient species, but variation cannot determine
their survival. It is at this stage that natural selection steps in.
But because natural selection allows certain mutations to persist, it is
not correct to say that natural selection has caused these mutations or
made or originated the species to which they give rise.
The Civil Service Commissioners do not make Indian civil servants: they
merely determine which of a number of ready-made men shall become civil
servants. Similarly, natural selection does not make new species, it
simply decides which of a number of ready-made organisms shall survive
and establish themselves as new species. Nor does natural selection
always do as much as this; for it is not the only determinant of
survival. Its position is sometimes comparable to that of the Medical
Board which inspects and rejects the physically unfit of the candidates
which have already been selected by some other authority.
The examination conducted by natural selection may be compared to a
competitive one. A separate, independent examination is held for each
particular locality; consequently the severity of the competition will
vary with the locality.
In each competition some candidates pass with ease: they gain an
unnecessarily high total of marks. So in nature do certain organisms, as,
for example, the Leaf-butterflies (_Kallimas_), appear to be over-adapted
to their environment. Other candidates manage to pass only by a very
narrow margin: these are paralleled in nature by those species which are
barely able to maintain themselves, which become extinct the moment the
competition increases in severity.
The great bulk of the candidates fail to obtain sufficient marks to gain
a place among the chosen few; these unsuccessful candidates correspond to
the mutating forms which perish in the struggle for existence, to those
individuals which happen to have mutated in unfavourable directions.
Even as many candidates have acquired knowledge of subjects in which they
are not examined, so do many organisms possess characteristics which are
of no utility to them in the struggle for existence.
Wallaceians expend much time and energy in misguided attempts to explain
the existence of such characters in terms of natural selection.
Nature’s examination, like that held for entrance to the Indian Civil
Service, is a liberal one, so that the qualifications of the successful
candidates vary considerably. Provided a candidate is able to gain more
marks than the other candidates for a vacancy, it matters not in what
subjects the marks are gained. So is it in nature. Natural selection
takes an organism as a whole. One species may have established itself
because of its fleetness, a second because of its courage, a third
because it has a strong constitution, a fourth because it is protectively
coloured, a fifth because it has good digestive powers, and so on.
We thus perceive the part played by natural selection and other forms of
isolation in the making of species. It is obvious that these do not make
species any more than the Civil Service Commissioners manufacture Indian
civil servants.
The real makers of species are the inherent properties of protoplasm and
the laws of variation and heredity. These determine the nature of the
organism; natural selection and the like factors merely decide for each
particular organism whether it shall survive and give rise to a species.
The way in which natural selection does its work is comparatively easy to
understand. But this is only the fringe of the territory which we call
evolution.
We seem to be tolerably near a solution of the problem of the causes of
the _survival_ of any particular mutation. This, however, is merely a
side issue. The real problem is the cause of variations and mutations,
or, in other words, how species _originate_. At present our knowledge of
the causes of variation and mutation is practically _nil_. We do not even
know along what particular lines mutations occur.
We have yet to discover whether one mutation invariably leads to another
along the same lines—in other words, whether mutating organisms behave as
though they had behind them a force acting in a definite direction. The
solution of these problems seems afar off. The hope of solving them lies,
not in the speculations in which biologists of to-day are so fond of
indulging, but in observation and experiment, especially the last.
The future of biology is largely in the hands of the practical breeder.
FOOTNOTES
[1]The white, pied, and “Japan” individuals are not more different from
the type than some variations occurring in wild birds.
[2]This short-legged type of dog is sometimes seen among the ownerless
and unselected pariah dogs of Indian towns; and a short-legged
variety of the fowl may occur sporadically in Zanzibar, where the
long-legged Malay is the prevalent breed.
[3]“Effected” appears in the earlier editions, but in the later editions
has given place to “affected,” probably a printer’s error.
[4]Some egrets, such as the rock-egrets (_Demiegretta_) of eastern
tropical coasts, are normally grey, but may be white, and this
whiteness may be confined in individuals to the young or adult
states.
[5]After years of observation of these Indian geese, Finn is convinced
they are now, at all events, pure Chinese; it is possible that they
really were hybrids in Blyth’s time, but that fresh importations of
geese from China, such as still occur, may have ultimately swamped
the blood of the common goose. The fertility of the hybrid geese was,
however, known to such early writers as Pallas and Linnæus. Darwin
himself, at a later date, bred five young from a pair of such hybrids
(_Nature_, Jan. 1, 1880, p. 207).
[6]In this chapter we use the word Neo-Darwinism in its usually-accepted
sense, _i.e._ as a name for that which should be called Wallaceism,
for the doctrine of the all-sufficiency of natural selection.
[7]_Animal Colouration_, p. 125. A book full of valuable facts and ideas
on this most interesting subject.
[8]Even these eggs, closely though they resemble in colouring the
shingle, etc., on which they are laid, are discovered and eaten by
gulls, as Mr A. J. R. Roberts points out in _The Bird Book_.
[9]_Journal of the Bombay Natural History Society_, Vol xv. (1903-4), p.
454.
[10]Hutton and Drummond record other examples of this in the valuable
work entitled _The Animals of New Zealand_.
INDEX
A
Accentor, 1
_Accipitcr cooperi_, 243
Acorn, 49
Acquired characters, 10, 14, 15, 18-24, 60, 107-10
_Acræeidæ_, 175, 215, 228
_Ægilops speltæformis_, 118
_Ægithina tiphia_, 244
Æsthetic sense in birds, 306
“African Nature Notes and Reminiscences,” 192, 195, 199
Aggressive resemblance, 173
Aguara-guazu, 181
Aitken, E. H., 64
“Albany Review, The,” 43, 48, 195, 204
Albinism, 64, 65, 99, 283, 284, 362
_Alcedo ispida_, 289
Alcock, Col., 216, 217
Alcohol, 152, 153
Alexander, 181
Allen, Grant, 66
Allotrophy, 159
Alternating characters, 143
Alternative inheritance, 127
Amadavat, 311
_Amandina erythrocephala_, 122
_A. fasciata_, 122
“Amazement,” 93
Amazon parrot, 103
Amazonian dolphin, 99
Ammonites, 67
Ammonium sulphate, 151
Amœba, 35
_Amphidasys betularia_, 101
_Anas boscas_, 123, 334
_A. obscura_, 334
_A. pœcilorhyncha_, 315, 334
_A. superciliosa_, 315, 334
_A. undulata_, 334
_Anastomus oscitans_, 282
Ancon sheep, 95
_Anemone magellanica_, 118
_A. sylvestris_, 118
Anemophilous flowers, 261
“Animal Colouration,” 194, 205, 211, 213, 218, 222
“Animal Life and Intelligence,” 368
“Animals of New Zealand,” 382
_Anous_, 278
_Anser cygnoides_, 114
_Anseranas melanoleucus_, 281
Antarctic fauna, 191
Antelope, 48, 199, 334
_Anthracoceros_, 220
_Anthropoides paradisea_, 279
_A. virgo_, 279
_Antilope cervicapra_, 363
Ape, 101
Apogamy, 370
Appenzeller, 340
_Apus_, 381
“Archiv für Entwicklungsmechanik der Organismen,” 325, 330
Arctic fauna, 173, 174, 190, 191
Arctic regions, 173, 189
_Ardea asha_, 317, 318
_A. gularis_, 318
_Ardeola grayii_, 250, 254
Argali, 120, 130, 131
“Argentine Ornithology,” 361
_Argynnis pales_, 381
_A. paphia_, 103
Aristotle, 1
_Artemia milhausenii_, 156
_A. salina_, 156
Aseel, 364
Asexual reproduction, 135
Asiatic, 140
Ass, 117, 127, 128, 140
_Astur badius_, 235
Atavism, 136, 293
_Athene chiaradiæ_, 97
_A. noctua_, 97
Atoms, biological, 158
“Auk, The,” 190
_Aularches militaris_, 216
Avebury, Lord, 205, 260
“Avicultural Magazine, The,” 98
Avocet, 80
B
Babbler, 244
Bactrian camel, 121
Bailey, 88
Baillon’s crake, 251
Balanced characters, 143
_Balearica chrysopelargus_, 105
_B. regulorum_, 105
_Bassaris astuta_, 242
Batesian mimicry, 177
Bateson, 26, 72, 73, 74, 75, 76, 102, 103, 302
Bats, 42
Bear, 101, 119, 190, 216, 282
Beddard, 180, 188, 194, 205, 211
Bee, 178, 179, 214, 221, 263, 264, 269
Beech, purple, 87
Bee-eater, 220, 278
Beetroots, 71
Belt, 216
Beluga, 190
Bentham, 260
Bestiary, 125
Bicheno’s finch, 105
Bilateral symmetry, 252, 253, 257
Bingham, Col. C. T., 239
Biological atoms, 158-69, 280
Biological molecules, 157-69, 280, 285, 291, 293, 294, 295, 344
Biological radicles, 158-69
Biophors, 153
“Bird Book, the,” 207
“Birds of the Plains,” 233, 303, 309, 359
Bison, 119, 126
Blackcock, 129, 131, 249, 278
Blackberry, 118
Blackbird, 201, 203, 207
Black-buck, 363
Blakiston, 181
Bloodsucker, 220
Blue-bellied waxbill, 104
Blyth, 115, 251
Boisier, 263
_Bombyx arrindia_, 125
_B. cynthia_, 124
Bonhote, 126, 288, 289, 290, 291, 292, 293, 337
Bontebock, 196
_Boselaphus tragocamelus_, 357, 363
_Bos frontalis_, 126
Boulenger, 88
Bower-bird, 306
Brain-fever bird, 235, 236, 248
Bramble, 261
_Branchipus_, 156
Brannam, 92
Brent, Mr, 307
British Museum, 129, 130, 187
_Bubo virginianus_, 221
_Bubulcus coromandus_, 254
Budgerigars, 101
Buffalo, 120, 199
Buffon, 2
Buff Orpingtons, 65
Buff-tip moth, 215
_Bufo melanostictus_, 219
Bulbul, 123, 220, 221, 244, 245, 255, 256, 279
Bull, 119
_Bungarus cœruleus_, 217, 247
Bunting, reed, 98, 190, 289
_Buphus coromandus_, 317, 318
Burbank, 118
Burnet moth, 102
Bush-buck, 196
Butcher-bird, 241, 253
Buttercups, 70, 267, 274
Butterfly, 45, 47, 102, 103, 196, 197, 203, 204, 209, 212, 216,
238, 239, 240, 250, 264, 280, 306, 381
Buzzard, 262
C
Cacomistle, 242
_Cairina moschata_, 127, 245
Californian currant, 119
_Calœnas nicobarica_, 65
_Calotes versicolor_, 220
Camel, 120, 357
_Campophaga_, 248
Canary, 100, 101, 102, 117, 120, 127, 280, 338, 362
_Canis jubatus_, 181
Capercailzie, 129, 131
Capuchin monkey, 216
Carbon, 153
_Carduelis caniceps_, 255
_C. carduelis_, 255
Carnation, 85, 86
Carnivores, 67
Carp, 102
Carrion crow, 123
Carrot, 71, 269, 270
_Casarca cana_, 129
_C. tadornoides_, 129
“Cassell’s Book of the Horse,” 69
Castle, 149
Castration, effects of, 335, 344
Cat, 61, 98, 99, 100, 206, 282, 283, 339, 350, 356, 361
Cat-rabbit, 125
Cataloe, 119
Cataract, 340
Caterpillars, 155, 175, 205, 211, 215, 221, 350
Cattle, 94, 95, 115
_Centropus sinensis_, 220, 244
_Cephalophus doriæ_, 243
_Cephalopyrus flammiceps_, 244
_Cervulus muntjac_, 101
_C. reevesii_, 114
_C. vaginalis_, 114
_Cervus paludosus_, 180
_C. sika_, 120
_Ceryle rudis_, 202
Chaffinch, 289
Chamba monaul, 104
“Champion Ladybird,” 91, 92, 93
Change of function, theory of, 36, 37
_Chen nivalis_, 282
_C. rossi_, 282
_Chenatopex ægyptiaca_, 316
_Chenonetta jubata_, 316
Chinese goose, 99, 114, 121, 130
Chinese pheasant, 123
_Chloëphaga dispar_, 105
_C. magellanica_, 105, 334
_C. rubidiceps_, 105, 334
Chromosomes, 145-7
_Chrysæna victor_, 333
_Chrysolophus amherstiæ_, 121
_C. obscurus_, 97
_C. pictus_, 97, 121, 337
_Chrysomitris colombiana_, 244
_Chrysotis æstiva_, 103
_Ciconia alba_, 282
_C. boyciana_, 282
Cinnabar moth, 227
_Cissopis leveriana_, 281
Civil Service Commissioners, 385, 387
Cleistogamous flowers, 260
Climate as check on multiplication, 349, 350
Clouded-yellow butterfly, 103
Clover, 69, 274
_Clytus arietis_, 178, 229
Cobra, 224, 225, 226, 358, 359
_Colias edusa_, 103
Colour-blindness, 340
Colouration of Flowers, Law of Progressive, 66
—— of Organisms, 170-296
Columbidæ, 331, 333
Concealing colouration, 184-7
Congenital characters, 18, 19
Conn, 47
“Contemporary Review,” 26
Cope, 15, 67
_Copsychus saularis_, 281
_Coracias affinis_, 123, 255
_C. indica_, 123, 220, 255
Cordon-bleu, 104
Cormorant, 190, 191, 277, 381, 382
Corn, Indian, 81
Correlation, 39, 40, 117, 162, 167, 223, 339, 340, 344, 356-65
_Corvus corone_, 123, 255
_C. cornix_, 123, 255
_C. splendens_, 353
“Country-Side, The,” 261, 265, 266, 273, 304, 311, 313
Courser, 362
Court-bec, 72
Cow, 119, 120, 126
Crab, 155
Crane, 105, 247, 248, 279, 282, 292
_Crateropus bicolor_, 242
_C. canorus_, 179
_Crax globicera_, 104, 304
_C. grayi_, 104
_C. hecki_, 104, 304
Crested newt, 124
Cretaceous reptiles, 67
Crinoids, 67
Crocodile, 187
Cross-fertilisation, 69, 258-60
_Crotalus_, 223
Crow, 47, 123, 206, 220, 247, 255, 281, 353, 355, 359, 361
“Crow-pheasant,” 220
Cryptic colouring, 173
Cuckoo, 220, 233, 235, 236, 243, 244, 247, 248, 289
—— shrike, 248
_Cuculus canorus_, 289
Cuénot, 149
Cunningham, Col., 225, 226, 358
—— J. T., 15, 19, 20, 324, 325, 329, 331, 332, 333, 336
Cupples, Mr, 308
Curassow, 104, 304
Currant, 119
Cut-throat finch, 122
_Cypselus affinis_, 243
_Cytisus adami_, 119
D
Dafila acuta, 122
Dahlia, 86
Daisy, 266, 274
Daltonism, 340
_Damaliscus lunatus_, 363
Damp as a check to multiplication of species, 350, 351
_Danaidæ_, 175, 179, 215, 216, 226, 228
_Danais chrysippus_, 179, 250
Danger signal, 183, 214, 253, 254
Darter, 277
Darwin, 1-12, 14, 25-27, 31, 35, 42, 52, 54-7, 59, 60-3, 68, 83,
96, 112, 114-7, 119, 123, 127, 130, 151, 171, 175, 182,
184, 233, 259, 299, 301-8, 316, 319-21, 325, 326, 347
“Darwin and after Darwin,” 370-5, 377, 381
Darwinian theory, 3, 5-8, 11, 13, 27, 28, 35, 42, 45, 52, 75, 111,
171
Darwinism, 1, 7, 8, 11, 14, 26
“Darwinism,” 40, 53, 112, 117, 178, 207, 213, 228, 322, 323
“Darwinism To-day,” 16, 45, 67
_Dasyurus_, 283
De Candolle, 86
Decorative plumage, 40
Deer, 101, 120, 180, 298
Deerhound, 304, 308
Deer-ponies, 125
Degeneration, 168
Dejerine, 340
Delage, 33, 147
Delbœuf, Law of, 373
_Delias eucharis_, 216, 220, 221
_Demiegretta_, 100
Demoiselle crane, 277
“Descent of Man,” 234, 299, 301, 302, 304, 305, 319, 320, 326
Determination of sex, 165
“Development and Heredity,” 17
De Vries, 26, 66, 69-72, 75-8, 82-9, 95, 105, 118, 151
Dewar, D., 43, 44, 47, 48, 195, 204, 206, 208, 210, 225, 233, 236,
303, 308, 309, 354, 360, 378
Dewar, G. A. B., 196, 197
_Dicrurus ater_, 179, 233
_Didelphys nurina_, 243
Dimorphism, sexual, 51, 200, 201
_Dipsacus_, 58
Disease as a check to multiplication of species, 351
_Dissemurus paradiseus_, 179, 220
Divergence of character, 367
Dog, 59, 68, 99, 100, 125, 225, 226, 282, 283, 304, 308, 352, 357,
364, 365
Dog-rose, 261
Dolphin, 99
Dominant characters, 142
Donald, Mr D., 256
Dragon-fly, 216, 264
Driesh, 136
Drongo cuckoo, 233
Drongo-shrikes, 179, 220
Drummond, 382
Duck, 51, 60, 68, 97, 99, 100, 122, 126-8, 190, 247, 249, 282, 292,
314, 315, 334, 337, 338, 365
Duiker-buck, 243
Dyer, Sir William Thistleton, 26
E
Eagle, 65, 190, 350
Eagle-owl, 221
East, M. E., 79
_Echis carinata_, 224
“Eclipse,” 69
“Edinburgh Review, The,” 38
Eel, 102
Eggs, colours of birds’, 206-9
Egret, 100, 206, 254, 365
Eider-duck, 249
Eimer, 15, 16, 33
Eisig, 222
_Elanoides furcatus_, 282
_Elaps_, 197, 198
Elder, 49
Elementary species, 77, 78, 87, 88, 89
Elk, Irish, 67
_Emberiza citrinella_, 289
_E. pyrrhuloides_, 98
_E. schœniclus_, 98
_Entomophila picata_, 281
Entomophilous flowers, 261
_Epenthesis folleata_, 103
Epilobias, 260
_Equus_, 41
_Erebia manto_, 381
_Erythrura prasina_, 102
“Essays on Evolution,” 11, 173, 177, 181, 184, 213, 223, 226, 227,
229, 230, 231, 234, 237, 238, 239
_Estrelda cyanogastra_, 104
_E. phœnicotis_, 104
Ether, 152, 153
_Euchelia jacobacæ_, 227
Eurasian, 140
European, 140
_Euxenura maguari_, 282
Evening primrose, 84, 85, 88
“Evolution of Sex, The,” 306
Existence, struggle for, 31, 32
Eye-colour in human beings, 310
Eyesight of birds, 211, 237-41
—— insects, 264
Eyton, 15
F
“Faery Year, The,” 196
Falcon, 204, 246, 250
_Falco peregrinator_, 251
_F. severus_, 251
False mimicry, 243
Faults in poultry, 64
Ferrets, 100, 119
Finch, 117, 120
—— Bicheno’s, 105
—— chestnut-breasted, 98
—— cut-throat, 122
—— Gouldian, 98
—— Nonpareil, 102
—— red-headed, 142
—— ringed, 104
—— saffron, 244
—— yellow-rumped, 98
Finn, 99, 102, 115, 131, 179, 216, 219, 220, 235, 241, 255, 304,
309, 310, 313, 315, 316, 358
Fittest, survival of the, 32
Flowers, 65, 66
Flowers, colours of, 258-75
Fly-catchers, 44, 45, 47, 285, 338
Flying squirrel, 243
“Fortnightly Review, The,” 37, 38
Foul-brood, 353
Fowl, 56, 58, 61, 64, 65, 99, 101, 125, 127, 128, 282, 301, 302,
307, 314, 330, 336, 338, 339, 361, 362, 364, 365
Fowl-ducks, 125
Foxes, 101, 131, 190, 191
Fox-terrier, 19
Franqueiro cattle, 95
_Francolinus pondicerianus_, 337
Friar-bird, 249
_Fringella coelebs_, 209
Fritillary butterfly, 103
Frog, 325
Fruits, colours of, 258, 275
_Fuligula marila_, 290
Fulmar petrel, 190
Function, change of, 36, 37
Fungi, 263
G
Gadow, Dr, 197, 245
Gadwall, 126, 315
Galton, 81, 82, 374
“Game Birds and Wild Fowl of India,” 131
Gametes, segregation of, 143-5
Gannet, 282
Gayal, 126
Gauchos, 359
Gecko, 210
Geddes, 306, 326
Gemmules, 151
“Genesis of Species,” 7, 61
Geographical isolation, 375
Geological record, imperfection of, 40-2, 94
Geranium, 260
Germ-plasm, continuity of the, 25
Germinal variations, 106-10
_Geum urbanum_, 263
Gibbon ape, 101
Giraffe, 17, 18, 192, 196
_Globicera_, 104
Glutton, 190
Goat, 283
Goethe, 2
Golden pheasant, 97, 129, 149, 337, 338
Golden tench, 101
Goldfinch, 127, 255
Goldfish, 101, 102
Goose, 99, 100, 105, 115, 121, 130, 190, 281, 316, 334, 339
Gordon’s currant, 119
Goshawk, 247
Gouldian Finch, 99
Graba, 58
Gradation of colour, principle of, 185
_Graculipica melanoptera_, 244
“Grammar of Science, The,” 309
Grass, 273
Grasshopper, 185
Greenfinch, 122
Greyhound, 364
Grosbeak, 281, 284
Groundsel, 260
Grouse, red, 125
Growth-force, 15, 16, 68
_Grus leucogeranus_, 282
Guillemot, 58, 190, 245
Guinea-fowl, 100, 127, 128, 279, 362
Guinea-pig, 95, 101, 129, 283
Gulick, 369, 372-7, 380
Gull, 190, 191, 207, 247, 290, 355
_Gygis_, 278
Gyrfalcon, 190
H
Haeckel, 15, 24
Hæmophilia, 340
_Halcyon smyrnensis_, 202
_Haliœtus albicilla_, 65
Hare, 131, 185, 200
Harrier, 101
Hartebeeste, 363
Hawk-cuckoo, 235, 236
Hawk-eagle, 101
Hawks, 222, 235, 236, 247, 277
_Hecki_, 104
Helice, 103
_Heliconidæ_, 175, 215, 216, 228
Heloderm, 217
Henslow, 15, 22, 23, 47, 48, 259
“Heredity,” 103, 145, 166, 340
“Heredity of Acquired Characters in Plants,” 22, 48
“Heredity of Sexual Characters in relation to Hormones,” 19, 330
Heron, 250, 317
Herring, 193
Hertwig, 151
Heusinger, 357
Hewitt, Mr, 307
_Hierococcyx varius_, 235, 248
Hilversum, 84
Himalayan argali, 120
Hinny, 127, 136, 140, 162
_Hipparchia, semele_, 205
_Hippotragus equinus_, 334
_H. niger_, 334
_Hirundo rustica_, 251
_H. tytleri_, 251
“History of Creation,” 24
Hobby, 250, 251
Homogamy, 370
Honeyeater, 281
Hormones, 335, 338
Hornbill, 65, 220
Horner, 340
Horse, 61, 68, 69, 95, 96, 100, 101, 117, 127, 128, 140, 266, 267,
268, 272, 283, 332, 352, 363, 364, 374
Horse, genealogy of, 41
Houghton, 91
Howard, 315, 332
Hubrecht, 26
Hume, 131
Humming-bird, 328
Hutton, 3
Hutton, Captain, 115, 382
Huxley, 3, 6, 11, 40, 100, 111
Hyæna, 353
Hybridism, 111-32, 292, 293
Hydra, 21
Hydrogen, 152, 153
_Hydrophasianus chirurgus_, 250
_Hyla_, 245
Hypertely, 237, 240
_Hypolimnas misippus_, 179, 180
I
“Ibis, The,” 255, 256
_Icterus vulgaris_, 244, 281, 284
Impeyan pheasant, 104
Indian Civil Service, 385, 386, 387
Indian corn, 81
Inheritance, 133-69
—— alternative, 127
—— blended, 140, 148
—— definition of, 138
—— of acquired characters, 10, 14, 15, 18-24, 60, 107-10
—— particulate, 140
—— unilateral, 139, 140, 162
Insectivores, 67
Intercrossing, swamping effects of, 42, 83
Intimidating attitudes, 224, 225
Iora, 244
Iridescence, 186
Irish elk, 67, 168
Isolation, 366-82, 387
Isomerism, biological, 154-8
—— chemical, 152-4, 157
_Ithomiinæ_, 228, 246
Ivy, 261
J
Jacana, 250
Jackdaw, 51, 306
Jaeger, 86
Jaguar, 45, 358
Japanese greenfinch, 122
—— pheasant, 122, 124, 129
Jardin des plantes, 88
Java sparrow, 99, 100
Jelly-fish, 192
Jesse, W., 255
Johnston, 92
“Journal of the Bombay Natural History Society,” 209
“Journal of the Royal Society of Arts,” 236, 324, 378
Jungle-babbler, 179
Jungle fowl, 332