man's unconscious selection, it seems to me almost certain that if the
individuals of one sex were during a long series of generations to
prefer pairing with certain individuals of the other sex, characterized
in some peculiar manner, the offspring would slowly but surely become
modified in this same manner. I have not attempted to conceal that,
excepting when the males are more numerous than the females, or when
polygamy prevails, it is doubtful how the more attractive males succeed
in leaving a larger number of offspring to inherit their superiority in
ornaments or other charms than the less attractive males; but I have
shown that this would probably follow from the females--especially the
more vigorous ones, which would be the first to breed--preferring not
only the more attractive but at the same time the more vigorous and
victorious males.
Although we have some positive evidence that birds appreciate bright and
beautiful objects, as with the bower-birds of Australia, and although
they certainly appreciate the power of song, yet I fully admit that it
is astonishing that the females of many birds and some mammals should be
endowed with sufficient taste to appreciate ornaments, which we have
reason to attribute to sexual selection; and this is even more
astonishing in the case of reptiles, fish and insects. But we really
know little about the minds of the lower animals. It cannot be supposed,
for instance, that male birds of paradise or peacocks should take such
pains in erecting, spreading and vibrating their beautiful plumes before
the males for no purpose. We should remember the fact given on excellent
authority in a former chapter that several peahens, when debarred from
an admired male, remained widows during a whole season rather than pair
with another bird.
Nevertheless, I know of no fact in natural history more wonderful than
that the female Argus pheasant should appreciate the exquisite shading
of the ball-and-socket ornaments and the elegant patterns on the wing
feathers of the male. He who thinks that the male was created as he now
exists must admit that the great plumes, which prevent the wings from
being used for flight and which, as well as the primary feathers, are
displayed in a manner quite peculiar to this one species during the act
of courtship, and at no other time, were given to him as an ornament. If
so, he must likewise admit that the female was created and endowed with
the capacity of appreciating such ornaments. I differ only in the
conviction that the male Argus pheasant acquired his beauty gradually,
through the females having preferred during many generations the more
highly ornamented males; the esthetic capacity of the females having
been advanced through exercise or habit just as our own taste is
gradually improved. In the male, through the fortunate chance of a few
feathers not having been modified, we can distinctly see how simple
spots with a little fulvous [tawny] shading on one side may have been
developed by small steps into the wonderful ball-and-socket ornaments;
and it is probable that they were actually thus developed.
Every one who admits the principle of evolution, and yet feels great
difficulty in admitting that female mammals, birds, reptiles and fish,
could have acquired the high taste implied by the beauty of the males,
and which generally coincides with our own standard, should reflect that
the nerve-cells of the brain in the highest as well as in the lowest
members of the Vertebrate series, are derived from those of the common
progenitor of the whole group. It thus becomes intelligible that the
brain and mental faculties should be capable under similar conditions of
nearly the same course of development, and consequently of performing
nearly the same functions.
The reader who has taken the trouble to go through the several chapters
devoted to sexual selection will be able to judge how far the
conclusions at which I have arrived are supported by sufficient
evidence. If he accepts these conclusions he may, I think, safely extend
them to mankind; but it would be superfluous here to repeat what I have
so lately said on the manner in which sexual selection apparently has
acted on man, both on the male and female side, causing the two sexes of
man to differ in body and mind, and the several races to differ from
each other in various characters, as well as from their ancient and
lowly organized progenitors.
He who admits the principle of sexual selection will be led to the
remarkable conclusion that the cerebral system not only regulates most
of the existing functions of the body, but has indirectly influenced the
progressive development of various bodily structures and of certain
mental qualities. Courage, pugnacity, perseverance, strength and size of
body, weapons of all kinds, musical organs, both vocal and instrumental,
bright colours, stripes and marks, and ornamental appendages, have all
been indirectly gained by the one sex or the other, through the
influence of love and jealousy, through the appreciation of the
beautiful in sound, colour or form, and through the exertion of a
choice; and those powers of the mind manifestly depend on the
development of the cerebral system.
Man scans with scrupulous care the character and pedigree of his horses,
cattle and dogs before he matches them; but when he comes to his own
marriage he rarely, or never takes any such care. He is impelled by
nearly the same motives as the lower animals when left to their own free
choice, though he is in so far superior to them that he highly values
mental charms and virtues. On the other hand he is strongly attracted
by mere wealth or rank. Yet he might by selection do something not only
for the bodily constitution and frame of his offspring, but for their
intellectual and moral qualities. Both sexes ought to refrain from
marriage if they are in any marked degree inferior in body or mind; but
such hopes are Utopian and will never be even partially realized until
the laws of inheritance are thoroughly known. All do good service who
aid toward this end. When the principles of breeding and inheritance are
better understood, we shall not hear ignorant members of our legislature
rejecting with scorn a plan for ascertaining whether or not
consanguineous marriages are injurious to man.
The advancement of the welfare of mankind is a most intricate problem;
all ought to refrain from marriage who cannot avoid abject poverty for
their children; for poverty is not only a great evil, but tends to its
own increase by leading to recklessness in marriage. On the other hand,
as Mr. Galton has remarked, if the prudent avoid marriage, while the
reckless marry, the inferior members tend to supplant the better members
of society. Man, like every other animal, has no doubt advanced to his
present high condition through a struggle for existence consequent on
his rapid multiplication; and if he is to advance still higher, he must
remain subject to a severe struggle. Otherwise he would sink into
indolence, and the more gifted men would not be more successful in the
battle of life than the less gifted. Hence our natural rate of
increase, though leading to many and obvious evils, must not be greatly
diminished by any means. There should be open competition for all men;
and the most able should not be prevented by laws or customs from
succeeding best and rearing the largest number of offspring. Important
as the struggle for existence has been and even still is, yet as far as
the highest part of man's nature is concerned there are other agencies
more important. For the moral qualities are advanced, either directly or
indirectly, much more through the effects of habit, the reasoning
powers, instruction, religion, etc., than through natural selection;
though to this latter agency the social instincts, which afforded the
basis for the development of the moral sense, may be safely attributed.
The main conclusion arrived at in this work, namely, that man is
descended from some lowly organized form, will, I regret to think, be
highly distasteful to many. But there can hardly be a doubt that we are
descended from barbarians. The astonishment I felt on first seeing a
party of Fuegians on a wild and broken shore will never be forgotten by
me, for the reflection at once rushed into my mind--such were our
ancestors. These men were absolutely naked and bedaubed with paint,
their long hair was tangled, their mouths frothed with excitement, and
their expression was wild, startled and distrustful. They possessed
hardly any arts, and like wild animals lived on what they could catch;
they had no government, and were merciless to every one not of their
own small tribe. He who has seen a savage in his native land will not
feel much shame, if forced to acknowledge that the blood of some more
humble creature flows in his veins. For my own part I would as soon be
descended from that heroic little monkey who braved his dreaded enemy in
order to save the life of his keeper; or from that old baboon, who,
descending from the mountains, carried away in triumph his young comrade
from a crowd of astonished dogs--as from a savage who delights to
torture his enemies, offers up bloody sacrifices, practises infanticide
without remorse, treats his wives like slaves, knows no decency, and is
haunted by the grossest superstitions.
Man may be excused for feeling some pride at having risen, though not
through his own exertions, to the very summit of the organic scale; and
the fact of his having thus risen, instead of having been aboriginally
placed there, may give him hope for a still higher destiny in the
distant future. But we are not here concerned with hopes or fears, only
with the truth as far as our reason permits us to discover it. I have
given the evidence to the best of my ability, and we must acknowledge,
as it seems to me, that man, with all his noble qualities, with sympathy
which feels for the most debased, with benevolence which extends not
only to other men but to the humblest living creature, with his godlike
intellect which has penetrated into the movements and constitution of
the solar system--with all these exalted powers--Man still bears in his
bodily frame the indelible stamp of his lowly origin.
MIMICRY AND OTHER PROTECTIVE RESEMBLANCES AMONG ANIMALS
ALFRED RUSSEL WALLACE
[Mr. Wallace, one of the greatest naturalists of the age,
discovered the law of natural selection independently of
Darwin, and about the same time. Among his works are "The
Malay Archipelago," "Island Life," and "Darwinism." From
"Natural Selection," which was published by Macmillan & Co.,
1871, the following extracts are taken. The theme has
received important development at the hands of Professor E.
B. Poulton, in his "The Colours of Animals," International
Scientific Series, 1890: and in F. E. Beddard's "Animal
Colouration"; London, Swan Sonnenschein; N. Y., Macmillan,
1892.]
There is no more convincing proof of the truth of a comprehensive
theory, than its power of absorbing and finding a place for new facts,
and its capability of interpreting phenomena which had been previously
looked upon as unaccountable anomalies. It is thus that the law of
universal gravitation and the undulatory theory of light have become
established and universally accepted by men of science. Fact after fact
has been brought forward as being apparently inconsistent with them, and
one after another these very facts have been shown to be the
consequences of the laws they were at first supposed to disprove. A
false theory will never stand this test. Advancing knowledge brings to
light whole groups of facts which it cannot deal with, and its advocates
steadily decrease in numbers, notwithstanding the ability and
scientific skill with which it has been supported. The course of a true
theory is very different, as may be well seen by the progress of opinion
on the subject of natural selection. In less than eight years "The
Origin of Species" has produced conviction in the minds of a majority of
the most eminent living men of science. New facts, new problems, new
difficulties as they arise are accepted, solved or removed by this
theory; and its principles are illustrated by the progress and
conclusions of every well established branch of human knowledge. It is
the object of the present essay to show how it has recently been applied
to connect together and explain a variety of curious facts which had
long been considered as inexplicable anomalies.
Perhaps no principle has ever been announced so fertile in results as
that which Mr. Darwin so earnestly impresses upon us, and which is
indeed a necessary deduction from the theory of natural selection,
namely--that none of the definite facts of organic nature, no special
organ, no characteristic form or marking, no peculiarities of instinct
or of habit, no relations between species or between groups of
species--can exist, but which must now be or once have been _useful_ to
the individuals or races which possess them. This great principle gives
us a clue which we can follow out in the study of many recondite
phenomena, and leads us to seek a meaning and a purpose of some definite
character in minutiae which we should be otherwise almost sure to pass
over as insignificant or unimportant.
The adaptation of the external colouring of animals to their conditions
of life has long been recognized, and has been imputed either to an
originally created specific peculiarity, or to the direct action of
climate, soil, or food. Where the former explanation has been accepted,
it has completely checked inquiry, since we could never get any further
than the fact of the adaptation. There was nothing more to be known
about the matter. The second explanation was soon found to be quite
inadequate to deal with all the varied phases of the phenomena, and to
be contradicted by many well-known facts. For example, wild rabbits are
always of gray or brown tints well suited for concealment among grass
and fern. But when these rabbits are domesticated, without any change of
climate or food, they vary into white or black, and these varieties may
be multiplied to any extent, forming white or black races. Exactly the
same thing has occurred with pigeons; and in the case of rats and mice,
the white variety has not been shown to be at all dependent on
alteration of climate, food or other external conditions. In many cases
the wings of an insect not only assume the exact tint of the bark or
leaf it is accustomed to rest on, but the form and veining of the leaf
or the exact rugosity of the bark is imitated; and these detailed
modifications cannot be reasonably imputed to climate or food, since in
many cases the species does not feed on the substance it resembles, and
when it does, no reasonable connection can be shown to exist between the
supposed cause and the effect produced. It was reserved for the theory
of natural selection to solve all these problems, and many others which
were not at first supposed to be directly connected with them. To make
these latter intelligible, it will be necessary to give a sketch of the
whole series of phenomena which may be classed under the head of useful
or protective resemblances.
Concealment, more or less complete, is useful to many animals, and
absolutely essential to some. Those which have numerous enemies from
which they cannot escape by rapidity of motion, find safety in
concealment. Those which prey upon others must also be so constituted as
not to alarm them by their presence or their approach, or they would
soon die of hunger. Now, it is remarkable in how many cases nature gives
this boon to the animal, by colouring it with such tints as may best
serve to enable it to escape from its enemies or to entrap its prey.
Desert animals as a rule are desert-coloured. The lion is a typical
example of this, and must be almost invisible when crouched upon the
sand or among desert rocks and stones. Antelopes are all more or less
sandy-coloured. The camel is pre-eminently so. The Egyptian cat and the
Pampas cat are sandy or earth-coloured. The Australian kangaroos are of
the same tints, and the original colour of the wild horse is supposed
to have been a sandy or clay-colour.
The desert birds are still more remarkably protected by their
assimilative hues. The stone-chats, the larks, the quails, the
goatsuckers and the grouse, which abound in the North African and
Asiatic deserts, are all tinted and mottled so as to resemble with
wonderful accuracy the average colour and aspect of the soil in the
district they inhabit. The Rev. H. Tristram, in his account of the
ornithology of North Africa in the first volume of the "Ibis," says: "In
the desert, where neither trees, brushwood, nor even undulation of the
surface afford the slightest protection to its foes, a modification of
colour which shall be assimilated to that of the surrounding country is
absolutely necessary. Hence _without exception_ the upper plumage of
_every bird_, whether lark, chat, sylvain, or sand-grouse, and also the
fur of _all the smaller mammals_, and the skin of _all the snakes and
lizards_, is of one uniform isabelline or sand colour." After the
testimony of so able an observer it is unnecessary to adduce further
examples of the protective colours of desert animals.
Almost equally striking are the cases of arctic animals possessing the
white colour that best conceals them upon snowfields and icebergs. The
polar bear is the only bear that is white, and it lives constantly among
snow and ice. The arctic fox, the ermine and the alpine hare change to
white in winter only, because in summer white would be more conspicuous
than any other colour, and therefore a danger rather than a protection;
but the American polar hare, inhabiting regions of almost perpetual
snow, is white all the year round. Other animals inhabiting the same
northern regions do not, however, change colour. The sable is a good
example, for throughout the severity of a Siberian winter it retains its
rich brown fur. But its habits are such that it does not need the
protection of colour, for it is said to be able to subsist on fruits and
berries in winter, and to be so active upon the trees as to catch small
birds among the branches. So also the woodchuck of Canada has a
dark-brown fur; but then it lives in burrows and frequents river banks,
catching fish and small animals that live in or near the water.
Among birds, the ptarmigan is a fine example of protective colouring.
Its summer plumage so exactly harmonizes with the lichen-coloured stones
among which it delights to sit, that a person may walk through a flock
of them without seeing a single bird; while in winter its white plumage
is an almost equal protection. The snow-bunting, the jerfalcon, and the
snowy owl are also white-coloured birds inhabiting the arctic regions,
and there can be little doubt but that their colouring is to some extent
protective.
Nocturnal animals supply us with equally good illustrations. Mice, rats,
bats, and moles possess the least conspicuous of hues, and must be quite
invisible at times when any light colour would be instantly seen. Owls
and goatsuckers are of those dark mottled tints that will assimilate
with bark and lichen, and thus protect them during the day, and at the
same time be inconspicuous in the dusk.
It is only in the tropics, among forests which never lose their foliage,
that we find whole groups of birds whose chief colour is green. The
parrots are the most striking example, but we have also a group of green
pigeons in the East; and the barbets, leaf-thrushes, bee-eaters,
white-eyes, turacos, and several smaller groups, have so much green in
their plumage as to tend greatly to conceal them among the foliage.
The conformity of tint which has been so far shown to exist between
animals and their habitations is of somewhat general character; we will
now consider the cases of more special adaptation. If the lion is
enabled by his sandy colour readily to conceal himself by merely
crouching down in the desert, how, it may be asked, do the elegant
markings of the tiger, the jaguar, and the other large cats agree with
this theory? We reply that these are generally cases of more or less
special adaptation. The tiger is a jungle animal, and hides himself
among tufts of grass or of bamboos, and in these positions the vertical
stripes with which his body is adorned must so assimilate with the
vertical stems of the bamboo, as to assist greatly in concealing him
from his approaching prey. How remarkable it is that besides the lion
and tiger, almost all the other large cats are arboreal in their
habits, and almost all have ocellated or spotted skins, which must
certainly tend to blend them with the background of foliage; while the
one exception, the puma, has an ashy-brown uniform fur, and has the
habit of clinging so closely to a limb of a tree while waiting for his
prey to pass beneath as to be hardly distinguishable from the bark.
Among birds, the ptarmigan, already mentioned, must be considered a
remarkable case of special adaptation. Another is a South American
goatsucker (Caprimulgus rupestris) which rests in the bright sunshine on
little bare rocky islets in the upper Rio Negro, where its unusually
light colours so closely resemble those of the rock and sand, that it
can scarcely be detected until trodden upon.
The Duke of Argyll, in his "Reign of Law," has pointed out the admirable
adaptation of the colours of the woodcock to its protection. The various
browns and yellows and pale ash-colour that occur on fallen leaves are
all reproduced in its plumage, so that when according to its habit it
rests upon the ground under trees, it is almost impossible to detect it.
In snipes the colours are modified so as to be equally in harmony with
the prevalent forms and colours of marshy vegetation. Mr. J. M. Lester,
in a paper read before the Rugby School Natural History Society
observes:--"The wood-dove, when perched amongst the branches of its
favourite _fir_, is scarcely discernible; whereas, were it among some
lighter foliage the blue and purple tints in its plumage would far
sooner betray it. The robin redbreast, too, although it might be thought
that the red on its breast made it much easier to be seen, is in reality
not at all endangered by it, since it generally contrives to get among
some russet or yellow fading leaves, where the red matches very well
with the autumn tints, and the brown of the rest of the body with the
bare branches."
Reptiles offer us many similar examples. The most arboreal lizards, the
iguanas, are as green as the leaves they feed upon, and the slender
whip-snakes are rendered almost invisible as they glide among the
foliage by a similar colouration. How difficult it is sometimes to catch
sight of the little green tree-frogs sitting on the leaves of a small
plant enclosed in a glass case in the Zoological Gardens; yet how much
better concealed they must be among the fresh green damp foliage of a
marshy forest. There is a North American frog found on lichen-covered
rocks and walls, which is so coloured as exactly to resemble them, and
as long as it remains quiet would certainly escape detection. Some of
the geckos which cling motionless on the trunks of trees in the tropics,
are of such curiously marbled colours as to match exactly with the bark
they rest upon.
In every part of the tropics there are tree snakes that twist among
boughs and shrubs, or lie coiled up in the dense masses of foliage.
These are of many distinct groups, and comprise both venomous and
harmless genera; but almost all of them are of a beautiful green colour,
sometimes more or less adorned with white or dusky bands and spots.
There can be little doubt that this colour is doubly useful to them,
since it will tend to conceal them from their enemies, and will lead
their prey to approach them unconscious of danger. Dr. Gunthner informs
me that there is only one genus of true arboreal snakes (Dipsas) whose
colours are rarely green, but are of various shades of black, brown, and
olive, and these are all nocturnal reptiles, and there can be little
doubt conceal themselves during the day in holes, so that the green
protective tint would be useless to them, and they accordingly retain
the more usual reptilian hues.