be a more perfectly developed and more highly organized form. It would
be in all respects better adapted to secure its safety, and to prolong
its individual existence and that of the race. Such a variety _could
not_ return to the original form; for that form is an inferior one, and
could never compete with it for existence. Granted, therefore, a
"tendency" to reproduce the original type of the species, still the
variety must ever remain preponderant in numbers, and under adverse
physical conditions _again alone survive_. But this new, improved, and
populous race might itself, in course of time, give rise to new
varieties, exhibiting several diverging modifications of form, any of
which, tending to increase the facilities for preserving existence,
must, by the same general law, in their turn become predominant. Here,
then, we have _progression and continued divergence_ deduced from the
general laws which regulate the existence of animals in a state of
nature, and from the undisputed fact that varieties do frequently occur.
It is not, however, contended that this result would be invariable; a
change of physical conditions in the district might at times materially
modify it, rendering the race which had been the most capable of
supporting existence under the former conditions now the least so, and
even causing the extinction of the newer and, for a time, superior race,
while the old or parent species and its first inferior varieties
continued to flourish. Variations in unimportant parts might also occur,
having no perceptible effect on the life-preserving powers; and the
varieties so furnished might run a course parallel with the parent
species, either giving rise to further variations or returning to the
former type. All we argue for is, that certain varieties have a tendency
to maintain their existence longer than the original species, and this
tendency must make itself felt; for though the doctrine of chances or
averages can never be trusted to on a limited scale, yet, if applied to
high numbers, the results come nearer to what theory demands, and, as we
approach to an infinity of examples, become strictly accurate. Now the
scale on which nature works is so vast--the numbers of individuals and
the periods of time with which she deals approach so near to infinity,
than any cause, however slight, and however liable to be veiled and
counteracted by accidental circumstances, must in the end produce its
full legitimate results.
_The Partial Reversion of Domesticated Varieties explained._
Let us now turn to domesticated animals, and inquire how varieties
produced among them are affected by the principles here enunciated. The
essential difference in the condition of wild and domestic animals is
this,--that among the former, their well-being and very existence depend
upon the full exercise and healthy condition of all their senses and
physical powers, whereas, among the latter, these are only partially
exercised, and in some cases are absolutely unused. A wild animal has to
search, and often to labour, for every mouthful of food--to exercise
sight, hearing, and smell in seeking it, and in avoiding dangers, in
procuring shelter from the inclemency of the seasons, and in providing
for the subsistence and safety of its offspring. There is no muscle of
its body that is not called into daily and hourly activity; there is no
sense or faculty that is not strengthened by continual exercise. The
domestic animal, on the other hand, has food provided for it, is
sheltered, and often confined, to guard it against the vicissitudes of
the seasons, is carefully secured from the attacks of its natural
enemies, and seldom even rears its young without human assistance. Half
of its senses and faculties become quite useless, and the other half are
but occasionally called into feeble exercise, while even its muscular
system is only irregularly brought into action.
Now when a variety of such an animal occurs, having increased power or
capacity in any organ or sense, such increase is totally useless, is
never called into action, and may even exist without the animal ever
becoming aware of it. In the wild animal, on the contrary, all its
faculties and powers being brought into full action for the necessities
of existence, any increase becomes immediately available, is
strengthened by exercise, and must even slightly modify the food, the
habits, and the whole economy of the race. It creates as it were a new
animal, one of superior powers, and which will necessarily increase in
numbers and outlive those which are inferior to it.
Again, in the domesticated animal all variations have an equal chance of
continuance; and those which would decidedly render a wild animal unable
to compete with its fellows and continue its existence are no
disadvantage whatever in a state of domesticity. Our quickly fattening
pigs, short-legged sheep pouter pigeons, and poodle dogs could never
have come into existence in a state of nature, because the very first
step towards such inferior forms would have led to the rapid extinction
of the race; still less could they now exist in competition with their
wild allies. The great speed but slight endurance of the race horse, the
unwieldy strength of the ploughman's team, would both be useless in a
state of nature. If turned wild on the pampas, such animals would
probably soon become extinct, or under favourable circumstances might
each gradually lose those extreme qualities which would never be called
into action, and in a few generations revert to a common type, which
must be that in which the various powers and faculties are so
proportioned to each other as to be best adapted to procure food and
secure safety,--that in which by the full exercise of every part of its
organisation the animal can alone continue to live. Domestic varieties,
when turned wild, _must_ return to something near the type of the
original wild stock, _or become altogether extinct_.[F]
+--------------------------------------------------------------+
| [F] That is, they will vary, and the variations which tend |
| to adapt them to the wild state, and therefore approximate |
| them to wild animals, will be preserved. Those individuals |
| which do not vary sufficiently will perish. |
+--------------------------------------------------------------+
We see, then, that no inferences as to the permanence of varieties in a
state of nature can be deduced from the observations of those occurring
among domestic animals. The two are so much opposed to each other in
every circumstance of their existence, that what applies to the one is
almost sure not to apply to the other. Domestic animals are abnormal,
irregular, artificial; they are subject to variations which never occur
and never can occur in a state of nature: their very existence depends
altogether on human care; so far are many of them removed from that just
proportion of faculties, that true balance of organisation, by means of
which alone an animal left to its own resources can preserve its
existence and continue its race.
_Lamarck's Hypothesis very different from that now advanced._
The hypothesis of Lamarck--that progressive changes in species have been
produced by the attempts of animals to increase the development of their
own organs, and thus modify their structure and habits--has been
repeatedly and easily refuted by all writers on the subject of varieties
and species, and it seems to have been considered that when this was
done the whole question has been finally settled; but the view here
developed renders such hypothesis quite unnecessary, by showing that
similar results must be produced by the action of principles constantly
at work in nature. The powerful retractile talons of the falcon-and the
cat-tribes have not been produced or increased by the volition of those
animals; but among the different varieties which occurred in the earlier
and less highly organized forms of these groups, _those always survived
longest which had the greatest facilities for seizing their prey_.
Neither did the giraffe acquire its long neck by desiring to reach the
foliage of the more lofty shrubs, and constantly stretching its neck for
the purpose, but because any varieties which occurred among its
antitypes with a longer neck than usual _at once secured a fresh range
of pasture over the same ground as their shorter-necked companions, and
on the first scarcity of food were thereby enabled to outlive them_.
Even the peculiar colours of many animals, more especially of insects,
so closely resembling the soil or leaves or bark on which they
habitually reside, are explained on the same principle; for though in
the course of ages varieties of many tints may have occurred, _yet those
races having colours best adapted to concealment from their enemies
would inevitably survive the longest_. We have also here an acting cause
to account for that balance so often observed in nature,--a deficiency
in one set of organs always being compensated by an increased
development of some others--powerful wings accompanying weak feet, or
great velocity making up for the absence of defensive weapons; for it
has been shown that all varieties in which an unbalanced deficiency
occurred could not long continue their existence. The action of this
principle is exactly like that of the centrifugal governor of the steam
engine, which checks and corrects any irregularities almost before they
become evident; and in like manner no unbalanced deficiency in the
animal kingdom can ever reach any conspicuous magnitude, because it
would make itself felt at the very first step, by rendering existence
difficult and extinction almost sure soon to follow. An origin such as
is here advocated will also agree with the peculiar character of the
modifications of form and structure which obtain in organized
beings--the many lines of divergence from a central type, the increasing
efficiency and power of a particular organ through a succession of
allied species, and the remarkable persistence of unimportant parts,
such as colour, texture of plumage and hair, form of horns or crests,
through a series of species differing considerably in more essential
characters. It also furnishes us with a reason for that "more
specialized structure" which Professor Owen states to be a
characteristic of recent compared with extinct forms, and which would
evidently be the result of the progressive modification of any organ
applied to a special purpose in the animal economy.
_Conclusion._
We believe we have now shown that there is a tendency in nature to the
continued progression of certain classes of _varieties_ further and
further from the original type--a progression to which there appears no
reason to assign any definite limits--and that the same principle which
produces this result in a state of nature will also explain why domestic
varieties have a tendency, when they become wild, to revert to the
original type. This progression, by minute steps, in various
directions, but always checked and balanced by the necessary conditions,
subject to which alone existence can be preserved, may, it is believed,
be followed out so as to agree with all the phaenomena presented by
organized beings, their extinction and succession in past ages, and all
the extraordinary modifications of form, instinct and habits which they
exhibit.
III.
MIMICRY, AND OTHER PROTECTIVE RESEMBLANCES AMONG ANIMALS.
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 phaenomena 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 may have been supported. The great name of Edward
Forbes did not prevent his theory of "Polarity in the distribution of
Organic beings in Time" from dying a natural death; but the most
striking illustration of the behaviour of a false theory is to be found
in the "Circular and Quinarian System" of classification propounded by
MacLeay, and developed by Swainson, with an amount of knowledge and
ingenuity that have rarely been surpassed. This theory was eminently
attractive, both from its symmetry and completeness, and from the
interesting nature of the varied analogies and affinities which it
brought to light and made use of. The series of Natural History volumes
in "Lardner's Cabinet Cyclopaedia," in which Mr. Swainson developed it in
most departments of the animal kingdom, made it widely known; and in
fact for a long time these were the best and almost the only popular
text-books for the rising generation of naturalists. It was favourably
received too by the older school, which was perhaps rather an indication
of its unsoundness. A considerable number of well-known naturalists
either spoke approvingly of it, or advocated similar principles, and for
a good many years it was decidedly in the ascendent. With such a
favourable introduction, and with such talented exponents, it must have
become established if it had had any germ of truth in it; yet it quite
died out in a few short years, its very existence is now a matter of
history; and so rapid was its fall that its talented creator, Swainson,
perhaps lived to be the last man who believed in it.
Such is the course of a false theory. That of a true one 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.
_Importance of the Principle of Utility._
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 the races which possess them. This great principle
gives us a clue which we can follow out in the study of many recondite
phaenomena, 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.
_Popular Theories of Colour in Animals._
The adaptation of the external colouring of animals to their conditions
of life has long been recognised, 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 phaenomena, and to
be contradicted by many well-known facts. For example, wild rabbits are
always of grey 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 to food, since
in many cases the species does not feed on the substance it resembles,
and when it does, no reasonable connexion 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 phaenomena which may be classed under the head of
useful or protective resemblances.
_Importance of Concealment as Influencing Colour._
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 stonechats, 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 1st volume of the "Ibis," says: "In
the desert, where neither trees, brush-wood, 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 jer-falcon, 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.
_Special Modifications of Colour._
The conformity of tint which has been so far shown to exist between
animals and their habitations is of a 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 upon 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 till 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 in 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."