it goes on too slowly, and our powers of observation are neither
comprehensive nor fine enough. How could we set about investigating
the millions of individuals which constitute the numerical strength
of a species on a given area, to find out whether they possess
some variable character in a definite percentage, and whether this
percentage increases in the course of decades or centuries? And
there is, furthermore, the difficulty of estimating the biological
importance of any variation that may occur. Even in cases where we know
its significance quite well in a general way, we cannot estimate its
relative value in reference to the variation of some other character,
though that other may also be quite intelligible. Later on, we shall
speak of protective colouring, and in so doing we shall discuss the
caterpillars of one of the Sphingidæ, which occur in two protective
colours, some being brown, others green. From the greater frequency of
the brown form we may conclude that brown is here a better adaptation
than green, but how could we infer this from the character itself,
or from our merely approximate knowledge of the mode of life of the
species, its habits, and the dangers which threaten it? A direct
estimation of the relative protective value of the two colours is
altogether out of the question. The survival of the fittest cannot be
proved in nature, simply because we are not in a position to decide,
_a priori_, what the fittest is. For this reason I was forced to try
to make the process of natural selection clear by means of imagined
examples, rather than observed ones.
But though we cannot directly follow the uninterrupted process of
natural selection which is going on under natural conditions, there is
another kind of proof for this hypothesis, besides that which consists
in logically deducing a process from correct premisses; I should like
to call this the practical proof. If a hypothesis can be made to
explain a great number of otherwise unintelligible facts, it thereby
gains a high degree of probability, and this is increased when there
are no facts to be found which are in contradiction to it.
Both of these criteria are fulfilled by the selection-hypothesis, and
indeed the phenomena which may be explained by it, and are intelligible
in no other way, present themselves to us in such enormous numbers,
that there can be no doubt whatever as to the correctness of the
principle; all that can be still disputed is, how far it reaches.
Let us now turn our attention to this practical way of proving the
theory by the facts which it serves to interpret, beginning with a
consideration of the external appearance of organisms, their colour and
form.
_The Colour and Form of Organisms._
Erasmus Darwin had in many cases already rightly recognized the
biological significance of the colouring of an animal species, and
we may be sure that many of the numerous good observers of earlier
times had similar ideas. I can even state definitely that Rösel von
Rosenhof, the famous miniature-painter and naturalist of Nürnberg in
the middle of the eighteenth century, recognized clearly, and gave
beautiful descriptions of what we now call colour-adaptation. It is
true that he gave them only as isolated instances, and was far from
recognizing the phenomenon of colour-adaptation in general, or even
from inquiring into its causes. From the time of Linné, the endeavour
to establish new species overshadowed all the finer observation of
life-habits and inter-relations, and, later on, after Blumenbach,
Kielmeyer, Cuvier, and others, the eager investigation of the internal
structure of animals also tended to divert attention from these
œcological relations. In systematic zoology, colour ranked only as a
diagnostic character of subordinate value, because it is often not very
stable, and indeed is sometimes very variable; it was therefore found
preferable to keep to such relatively stable differences as are to be
found in the form, size, and number of parts.
Charles Darwin was the first to redirect attention to the fact that the
colouring of animals is anything but an unimportant matter; that, on
the contrary, in many cases it is of use to the animal, e.g. in making
it inconspicuous; a green insect is not readily seen on green leaves,
nor a grey-brown one on the bark of a tree.
It is plain that the origin of such a so-called 'sympathetic'
coloration, harmonizing with the usual environment of the animal, can
be easily interpreted in terms of the principle of selection; and it is
equally evident that it cannot be explained by the Lamarckian principle
of transformation. Through the accumulation of slight useful variations
in colour, it is quite possible for a green or a brown insect to arise
from a previous colour, but a grey or a brown insect could not possibly
have become a green one simply by getting into the habit of sitting on
a green leaf; and still less can the will of the animal or any kind
of activity have brought the change about. Even if the animal had any
idea that it would be very useful to it to be coloured green, now that
it had got into the habit of sitting on a leaf, it could not have done
anything towards attaining the desirable green colour. Quite recently
the possibility of a kind of colour-photography on the skin of the
animal has been suggested, but there are many species whose colouring
is in contrast to their environment, so that the skin in these cases
does not act as a photographic plate, and it would, therefore, have
to be explained how it comes to pass that it functions as such in
the sympathetically coloured animals. I do not ask for proof of the
chemical composition of the stuff which is supposed to be sensitive to
light. Whether this be iodide of silver or something quite different,
the question remains the same: how comes it that it has only appeared
in animals to which a sympathetic colouring is advantageous in the
struggle for life? And the answer, from our point of view, must read:
it has arisen through natural selection in those species to which a
sympathetic colouring is useful. Thus even if the supposition that
sympathetic colouring is due to automatic photography on the part of
the skin were correct, we should still have to regard it as an outcome
of natural selection; but it is not correct--at least in general--as
the above objection shows, and as will be further apparent from many of
the phenomena of colour-adaptation which I shall now adduce.
To explain sympathetic coloration, then, we must assume, with Darwin
and Wallace, a process of selection due to the fact that, as changes
took place in the course of time in the colouring of the surroundings,
those individuals on an average most easily escaped the persecution of
their enemies which diverged least in colour from their surroundings,
and so, in the course of generations, an ever greater harmony with this
colouring was established. Variations in colouring crop up everywhere,
and as soon as these reached such a degree as to afford their
possessors a more effective protection than the colouring of their
fellows, then natural selection of necessity stepped in, and would only
cease to act when the harmony with the environment had become complete,
or, at least, so nearly so that any increase of it could not heighten
the deception.
Of course, it is presupposed in the working out this selective process
that the species has enemies which see. This is the case, however, with
most animals living on the earth or in the water, unless they are of
microscopic minuteness. Many animals, too, are subject to persecution
not only in their adult state, but at almost every period of their
life, and so, in general, we should expect that many of them would have
attained at each stage that coloration of body that would render them
least liable to discovery by their enemies.
And this is in reality the case: numerous animals are protected in some
measure by so-called sympathetic colouring, from the egg to the adult
state.
Let us begin with the egg, and of course there is no need to speak of
any eggs except those which are laid. Of these many are simply white
in colour, e.g. the eggs of many birds, snakes, and lizards, and this
seems to contradict our prediction; but these eggs are either hidden in
earth, compost, or sand, as in the case of the reptiles, or they are
laid in dome-shaped nests, or concealed in holes in trees, as in many
birds; thus they require no protective colouring.
In other cases, however, numerous eggs, especially of insects and
birds, possess a colouring which makes it very difficult to distinguish
them from their usual surroundings. Our large green grasshopper
(_Locusta viridissima_) lays its eggs in the earth, and they are
brown, exactly like the earth which surrounds them. They are enough
in themselves to refute the hypothesis that sympathetic colouring has
arisen through self-photography, for these eggs lie in total darkness
in the ground. Insect-eggs which are laid on the bark of trees are
often grey-brown or whitish like it, and the eggs of the humming-bird
hawk-moth (_Macroglossa stellatarum_), which are attached singly to the
leaves of the bedstraw, have the same beautiful light-green colour as
these leaves, and, in point of fact, green is a predominant colour of
the eggs in a very large number of insects.
But the eggs of many birds, too, exhibit 'sympathetic' colouring; thus
the curlew (_Numenius arquata_) has green eggs, which are laid in the
grass; but the red grouse (_Lagopus scoticus_) lays blackish-brown
eggs, exactly of the colour of the surrounding moor-soil; and it has
been observed that they remain uncovered for twelve days, for the
hen lays only one egg daily, and does not begin to brood until the
whole number of twelve is complete. Herein lies the reason of the
colour-adaptation, which the eggs would not have required, if they had
always been covered by the brooding bird.
The eggs of birds are frequently not of one colour only; those of the
Alpine ptarmigan (_Lagopus alpinus_), for instance, are ochre-yellow
with brown and red-brown dots, resembling the nest, which is carelessly
constructed of dry parts of plants. Sometimes this mingling of colours
reaches an astonishing degree of resemblance to surroundings, as in the
golden plover (_Charadrius pluvialis_), whose eggs, like those of the
peewit (_Vanellus cristatus_), are laid among stones and grasses, not
in a true nest, but in a flat depression in the sand, and, protected
by a motley speckling with streaking of white, yellow, grey and brown,
are excellently concealed. Perhaps the eggs of the sandpipers and gulls
are even better protected, for their colouring is a mingling of yellow,
brown, and grey, which imitates the sand in which they are laid so
perfectly, that one may easily tread on them before becoming aware of
them.
But let us now turn from eggs to adult animals. Darwin first pointed
out that the fauna of great regions may exhibit one and the same
ground-colouring, as is the case in the Arctic zone and in the deserts.
The most diverse inhabitants of these regions show quite similar
coloration, namely, that which harmonizes with the dominant colour of
the region itself. It is not only the persecuted animals, which need
protection, that are sympathetically coloured in these cases, the
persecutors themselves are likewise adapted, and this need not surprise
us, when we remember that the very existence of a beast of prey
depends on its being able to gain possession of its victims, and that
therefore it must be of the greatest use to it to contrast as little as
possible with its surroundings, and thus be able to steal on its quarry
unperceived. Those that are best adapted in colour will secure the most
abundant food, and will reproduce most prolifically; and they will thus
have a better prospect of transmitting their usual colouring to their
offspring. The Polar bear would starve if he were brown or grey, like
his relatives; among the ice and snow of the Polar regions his victims,
the seals, would see him coming from afar.
In the Arctic zone the adaptation of the colouring of the animals to
the white of the surroundings is particularly striking. Most of the
mammals there are pure white, or approximately white, at least during
the long winter; and it is easily understood that they must be so if
they are to survive in the midst of the snow and ice,--both beasts of
prey and their victims. For the latter the sympathetic colouring is of
'protective' value; for the former, of 'aggressive' value (Poulton).
Thus we find not only the Polar hare and the snow-bunting white, but
also the Arctic fox, the Polar bear, and the great snowy owl; and
though the brown sable is an exception, that is intelligible enough,
for he lives on trees, and is best concealed when he cowers close to
the dark trunk and branches. For him there would be no advantage in
being white, and therefore he has not become so.
Desert animals are also almost all sympathetically coloured, that
is, they are of a peculiarly sandy yellow, or yellowish-brown, or
clayey-yellow, or a mixture of all these colours; and here again the
beasts of prey and their victims are similarly coloured. The lion must
be almost invisible from a short distance, when he steals along towards
his prey, crouching close to the ground; but the camel too, the various
species of antelope, the giraffe, all the smaller mammals, and also the
horned viper (_Vipera cerastes_), the Egyptian spectacled snake (_Naja
haje_), many lizards, geckos, and the great Varanus, numerous birds,
not a few insects, especially locusts, show the colours of the desert.
It is true that the birds often have very conspicuous colours, such as
white on breast and under parts, but the upper surface is coloured like
the desert, and conceals them from pursuers whenever they cower close
to the ground. It has even been observed that a locust of the genus
_Tryxalis_ is of a light sand-colour in the sandy part of the Libyan
desert, but dark brown in its rocky parts, thus illustrating a double
adaptation in the same species.
Another group, which agrees in colour with the general surroundings,
is that of the 'glass-animals,' as they have been called, though
perhaps 'crystal animals' is a better term. A great number of simple
free-swimming marine forms, and a few fresh-water ones, are quite
colourless, and perfectly transparent, or have at most a bluish or
greenish tinge, and on this account they are quite invisible as long as
they remain in the water. In our lakes there lives a little crustacean
about a centimetre in length, of the order of water-fleas (_Leptodora
hyalina_), a mighty hunter among the smallest animals, which swims
forward jerkily with its long swimming-appendages, and widely spreads
its six pairs of claws, armed with thorny bristles, like a weir basket,
to seize its prey. We may have dozens of these in a glass of water
without being able to see a single one, even when we hold the glass
against the light, for the creatures are crystal-clear and transparent,
and have exactly the same refractive power as the water. It requires a
very sharp scrutiny and a knowledge of the animals to be able to detect
in the water little yellowish stripes, which are the stomachs of the
animals filled with food in process of digestion, for which, as we can
readily understand, invisibility cannot very well be arranged. If the
water be then strained through a fine cloth, a little gelatine-like
mass of the bodies of the _Leptodora_ will remain on the sieve.
A great many of the lower marine animals are equally transparent,
and as clear as water; most of the lower Medusæ, the ctenophores,
various molluscs, the barrel-shaped Salpæ, worms, many crustaceans of
quite different orders, and above all an enormous number of larvæ of
the most diverse animal groups. I can remember seeing the sea at the
shore at Mentone so full of Salpæ, that in every glass of sea-water
drawn at random there were many of them, and sometimes a glass held a
positive animal soup. But one did not see them in the glass of water,
and only those who knew what to look for recognized them by the bluish
intestinal sac that lies posteriorly in the invisible body. But when
the water was poured off through a fine net, there remained on the
filter a large mass of a crystalline gelatinous substance.
It is obvious that this must serve as a protective arrangement, for the
animals are not seen by their pursuers; but it is not an _absolute_
protection, for they have many pursuers who do not wait till they see
their prey, but are almost constantly snapping the mouth open and
shut, leaving it to chance to bring them their prey. _No protective
arrangement, however, affords absolute security_; it protects against
some enemies, perhaps against many, but never against all.
But now let us turn to a group of a different colouring, the green
animals. We are familiar with our big grass-green grasshopper, and
we know how easily it is overlooked when it sits quietly on a high
grass-stem, surrounded by grasses and herbage; the light grass-green of
its whole body protects it most effectively from discovery: for myself,
at least, I must confess that in a flowery meadow I have stood right
in front of one, and have looked close to it for a long time without
detecting it. In the same way countless insects of the most diverse
groups--bugs, dipterous flies, sawflies, butterflies--and especially
the larvæ (caterpillars) of the last, are of the same green as the
plants on which they live, and this again applies to the predaceous
species, as well as the species preyed upon. Thus the rapacious
praying-mantis (_Mantis religiosa_) is as green as the grass in which
it lurks motionless for its victim--a dragonfly, a fly, or a butterfly.
There are also green spiders, green amphibians like the edible frog,
and especially the tree-frog, green reptiles like lizards and the
tree-snakes of tropical forests. It is always animals which live among
green that are green in colour.
We may wonder, for a moment, why there are so few green birds, since
they spend so much of their time among the green leaves. But this
paucity of green birds is only true of temperate climates. In Germany
we have only the green woodpecker, the siskin, and a few other little
birds, and even these are not of a bright green, but are rather
greyish-green. The explanation lies in the long winter, when the
trees are leafless. In the evergreen forests of the tropics there are
numerous green birds belonging to very diverse families.
Yet another group with a common colour-adaptation deserves mention--the
beasts of the night. They are all more or less grey, brown, yellowish,
or a mixture of these colours, and it is obvious that, in the
duskiness of night, they must blend better with their environment on
this account. White mice and white rats cannot exist under natural
conditions, since they are conspicuous in the night, and the same would
be true of white bats, nightjars, and owls; but all of these have a
coloration suited to nocturnal habits.
A very remarkable fact is that in many animals the colour-adaptation
is a double one. Thus the Arctic fox is white only in winter, while in
summer he is greyish-brown; the ermine changes in the same way, and the
great white snowy owl of the Arctic regions has in summer a grey-brown
variegated plumage. Many animals which are subject to persecution
also change colour with the seasons, like the mountain hare (_Lepus
variabilis_), which is brown in summer and pure white in winter, the
Lapland lemming, and the ptarmigan (_Lagopus alpinus_), which do the
same. It has been doubted whether natural selection can explain this
double coloration, but I do not know where the difficulty lies, and
there is certainly no other principle whose aid we can evoke. The
mountain hare must have had some sort of colour before it attained to
seasonal dimorphism. Let us assume that it was brown, that the climate
became colder and the winter longer, then those hares would have most
chance of surviving which became lighter in winter, and so a white race
was formed. Poulton has shown that the whiteness is due to the fact
that the dark hairs of the summer coat grow white as they lengthen at
the beginning of winter, and the abundance of new hairs which complete
the winter coat are from the first white throughout. If the white hairs
were to persist throughout the summer it would be very disadvantageous
to their wearer; so a double selection must take place, in summer the
individuals which remain white, in winter those which remain brown,
being most frequently eliminated, so that only those would be left
which were brown in summer and white in winter. This double selection
would be favoured by the fact that there would be, in any case, a
change of fur at the beginning of summer; the winter hairs fall out
and the fur becomes thinner. The process does not differ essentially
from that which takes place in any species when two or more parts or
characters, which are not directly connected, have to be changed, such
as, for instance, colour and fertility. The struggle for existence will
in this case be favourable, on the one hand, to the advantageously
coloured, and on the other to the most fertile, and though the two
characters may at first only occur separately, they will soon be united
by free crossing, until ultimately only those individuals will occur
which are at once the most favourably coloured and the most fertile.
So in this case there remain only those which are brown in summer and
white in winter.
We must ascribe to the influence of the processes of selection the
exact regulation of the duration of the winter and summer dress, which
has been carefully studied in the case of the variable hare. In the
high Alps it remains white for six or seven months, in the south of
Norway for eight months, in Northern Norway for nine months, and in
Northern Greenland it never loses its white coat at all, as there
the snow, even in summer, melts only in some places and for a short
time. But apart from concealment there is certainly another adaptation
involved here--namely, the growth of the hair as a protection against
the cold. From an old experiment made in 1835 by Captain J. Ross, and
recently brought to light again by Poulton, we learn that a captive
lemming kept in a room in winter did not change colour until it was
exposed to the cold. The constitution of animals which become white in
winter is thus so organized that the setting in of cold weather acts
as a stimulus which incites the skin to the production of white hairs.
This predisposition also we must refer to the influence of natural
selection, since it must have been very useful to the species that the
winter coat should grow just when it was necessary as a protection
against cold. This explains at the same time why the predisposition
to respond to the stimulus of cold by a growth of winter fur finds
expression earlier in those colonies of Arctic animals, such as the
hare, which live in Lapland, than in those which live in the south of
Norway.
But that it is not the _direct_ influence of cold which colours the
hair of a furred animal white we can see from our common hare (_Lepus
timidus_), which, in spite of the winter's cold, does not become white,
but retains its brown coat, and not less so from the mountain hare
(_Lepus variabilis_), which in the south of Sweden also remains brown,
although the winter there may be exceedingly cold. But as the covering
of the ground with snow is not so uninterrupted there as in the higher
North, a white coat would be not a better protection than a brown
one, but a worse. The white colouring of Arctic animals is therefore
not directly due to the influence of the climate, as has often been
maintained, but is due to it indirectly, that is, through the operation
of natural selection. I have tried to make this clear by means of this
example, so that we may not have to repeat it in considering those
which are to follow.
* * * * *