food, or to escape from beasts of prey.
Organs now of trifling importance have probably in some cases been of high
importance to an early progenitor, and, after having been slowly perfected
at a former period, have been transmitted in nearly the same state,
although now become of very slight use; and any actually injurious
deviations in their structure will always have been checked by natural
selection. Seeing how important an organ of locomotion the tail is in most
aquatic animals, its general presence and use for many purposes in so many
land animals, which in their lungs or modified swim-bladders betray their
aquatic origin, may perhaps be thus accounted for. A well-developed tail
having been formed in an aquatic animal, it might subsequently come to be
worked in for all sorts of purposes, as a fly-flapper, an organ of
prehension, or as an aid in turning, as with the dog, though the aid must
be slight, for the hare, with hardly any tail, can double quickly enough.
In the second place, we may sometimes attribute importance to characters
which are really of very little importance, and which have originated from
quite secondary causes, independently of natural selection. We should
remember that climate, food, &c., probably have some little direct
influence on the organisation; that characters reappear from the law of
reversion; that correlation of growth will have had a most important
influence in modifying various structures; and finally, that sexual
selection will often have largely modified the external characters of
animals having a will, to give one male an advantage in fighting with
another or in charming the females. Moreover when a modification of
structure has primarily arisen from the above or other unknown causes, it
may at first have been of no advantage to the species, but may subsequently
have been taken advantage of by the descendants of the species under new
conditions of life and with newly acquired habits.
To give a few instances to illustrate these latter remarks. If green
woodpeckers alone had existed, and we did not know that there were many
black and pied kinds, I dare say that we should have thought that the green
colour was a beautiful adaptation to hide this tree-frequenting bird from
its enemies; and consequently that it was a character of importance and
might have been acquired through natural selection; as it is, I have no
doubt that the colour is due to some quite distinct cause, probably to
sexual selection. A trailing bamboo in the Malay Archipelago climbs the
loftiest trees by the aid of exquisitely constructed hooks clustered around
the ends of the branches, and this contrivance, no doubt, is of the highest
service to the plant; but as we see nearly similar hooks on many trees
which are not climbers, the hooks on the bamboo may have arisen from
unknown laws of growth, and have been subsequently taken advantage of by
the plant undergoing further modification and becoming a climber. The
naked skin on the head of a vulture is generally looked at as a direct
adaptation for wallowing in putridity; and so it may be, or it may possibly
be due to the direct action of putrid matter; but we should be very
cautious in drawing any such inference, when we see that the skin on the
head of the clean-feeding male turkey is likewise naked. The sutures in
the skulls of young mammals have been advanced as a beautiful adaptation
for aiding parturition, and no doubt they facilitate, or may be
indispensable for this act; but as sutures occur in the skulls of young
birds and reptiles, which have only to escape from a broken egg, we may
infer that this structure has arisen from the laws of growth, and has been
taken advantage of in the parturition of the higher animals.
We are profoundly ignorant of the causes producing slight and unimportant
variations; and we are immediately made conscious of this by reflecting on
the differences in the breeds of our domesticated animals in different
countries,--more especially in the less civilized countries where there has
been but little artificial selection. Careful observers are convinced that
a damp climate affects the growth of the hair, and that with the hair the
horns are correlated. Mountain breeds always differ from lowland breeds;
and a mountainous country would probably affect the hind limbs from
exercising them more, and possibly even the form of the pelvis; and then by
the law of homologous variation, the front limbs and even the head would
probably be affected. The shape, also, of the pelvis might affect by
pressure the shape of the head of the young in the womb. The laborious
breathing necessary in high regions would, we have some reason to believe,
increase the size of the chest; and again correlation would come into play.
Animals kept by savages in different countries often have to struggle for
their own subsistence, and would be exposed to a certain extent to natural
selection, and individuals with slightly different constitutions would
succeed best under different climates; and there is reason to believe that
constitution and colour are correlated. A good observer, also, states that
in cattle susceptibility to the attacks of flies is correlated with colour,
as is the liability to be poisoned by certain plants; so that colour would
be thus subjected to the action of natural selection. But we are far too
ignorant to speculate on the relative importance of the several known and
unknown laws of variation; and I have here alluded to them only to show
that, if we are unable to account for the characteristic differences of our
domestic breeds, which nevertheless we generally admit to have arisen
through ordinary generation, we ought not to lay too much stress on our
ignorance of the precise cause of the slight analogous differences between
species. I might have adduced for this same purpose the differences
between the races of man, which are so strongly marked; I may add that some
little light can apparently be thrown on the origin of these differences,
chiefly through sexual selection of a particular kind, but without here
entering on copious details my reasoning would appear frivolous.
The foregoing remarks lead me to say a few words on the protest lately made
by some naturalists, against the utilitarian doctrine that every detail of
structure has been produced for the good of its possessor. They believe
that very many structures have been created for beauty in the eyes of man,
or for mere variety. This doctrine, if true, would be absolutely fatal to
my theory. Yet I fully admit that many structures are of no direct use to
their possessors. Physical conditions probably have had some little effect
on structure, quite independently of any good thus gained. Correlation of
growth has no doubt played a most important part, and a useful modification
of one part will often have entailed on other parts diversified changes of
no direct use. So again characters which formerly were useful, or which
formerly had arisen from correlation of growth, or from other unknown
cause, may reappear from the law of reversion, though now of no di
rect use.
The effects of sexual selection, when displayed in beauty to charm the
females, can be called useful only in rather a forced sense. But by far
the most important consideration is that the chief part of the organisation
of every being is simply due to inheritance; and consequently, though each
being assuredly is well fitted for its place in nature, many structures now
have no direct relation to the habits of life of each species. Thus, we
can hardly believe that the webbed feet of the upland goose or of the
frigate-bird are of special use to these birds; we cannot believe that the
same bones in the arm of the monkey, in the fore leg of the horse, in the
wing of the bat, and in the flipper of the seal, are of special use to
these animals. We may safely attribute these structures to inheritance.
But to the progenitor of the upland goose and of the frigate-bird, webbed
feet no doubt were as useful as they now are to the most aquatic of
existing birds. So we may believe that the progenitor of the seal had not
a flipper, but a foot with five toes fitted for walking or grasping; and we
may further venture to believe that the several bones in the limbs of the
monkey, horse, and bat, which have been inherited from a common progenitor,
were formerly of more special use to that progenitor, or its progenitors,
than they now are to these animals having such widely diversified habits.
Therefore we may infer that these several bones might have been acquired
through natural selection, subjected formerly, as now, to the several laws
of inheritance, reversion, correlation of growth, &c. Hence every detail
of structure in every living creature (making some little allowance for the
direct action of physical conditions) may be viewed, either as having been
of special use to some ancestral form, or as being now of special use to
the descendants of this form--either directly, or indirectly through the
complex laws of growth.
Natural selection cannot possibly produce any modification in any one
species exclusively for the good of another species; though throughout
structure of another. But natural selection can and does often produce
structures for the direct injury of other species, as we see in the fang of
the adder, and in the ovipositor of the ichneumon, by which its eggs are
deposited in the living bodies of other insects. If it could be proved
that any part of the structure of any one species had been formed for the
exclusive good of another species, it would annihilate my theory, for such
could not have been produced through natural selection. Although many
statements may be found in works on natural history to this effect, I
cannot find even one which seems to me of any weight. It is admitted that
the rattlesnake has a poison-fang for its own defence and for the
destruction of its prey; but some authors suppose that at the same time
this snake is furnished with a rattle for its own injury, namely, to warn
its prey to escape. I would almost as soon believe that the cat curls the
end of its tail when preparing to spring, in order to warn the doomed
mouse. But I have not space here to enter on this and other such cases.
Natural selection will never produce in a being anything injurious to
itself, for natural selection acts solely by and for the good of each. No
organ will be formed, as Paley has remarked, for the purpose of causing
pain or for doing an injury to its possessor. If a fair balance be struck
between the good and evil caused by each part, each will be found on the
whole advantageous. After the lapse of time, under changing conditions of
life, if any part comes to be injurious, it will be modified; or if it be
not so, the being will become extinct, as myriads have become extinct.
Natural selection tends only to make each organic being as perfect as, or
slightly more perfect than, the other inhabitants of the same country with
which it has to struggle for existence. And we see that this is the degree
of perfection attained under nature. The endemic productions of New
Zealand, for instance, are perfect one compared with another; but they are
now rapidly yielding before the advancing legions of plants and animals
introduced from Europe. Natural selection will not produce absolute
perfection, nor do we always meet, as far as we can judge, with this high
standard under nature. The correction for the aberration of light is said,
on high authority, not to be perfect even in that most perfect organ, the
eye. If our reason leads us to admire with enthusiasm a multitude of
inimitable contrivances in nature, this same reason tells us, though we may
easily err on both sides, that some other contrivances are less perfect.
Can we consider the sting of the wasp or of the bee as perfect, which, when
used against many attacking animals, cannot be withdrawn, owing to the
backward serratures, and so inevitably causes the death of the insect by
tearing out its viscera?
If we look at the sting of the bee, as having originally existed in a
remote progenitor as a boring and serrated instrument, like that in so many
members of the same great order, and which has been modified but not
perfected for its present purpose, with the poison originally adapted to
cause galls subsequently intensified, we can perhaps understand how it is
that the use of the sting should so often cause the insect's own death:
for if on the whole the power of stinging be useful to the community, it
will fulfil all the requirements of natural selection, though it may cause
the death of some few members. If we admire the truly wonderful power of
scent by which the males of many insects find their females, can we admire
the production for this single purpose of thousands of drones, which are
utterly useless to the community for any other end, and which are
ultimately slaughtered by their industrious and sterile sisters? It may be
difficult, but we ought to admire the savage instinctive hatred of the
queen-bee, which urges her instantly to destroy the young queens her
daughters as soon as born, or to perish herself in the combat; for
undoubtedly this is for the good of the community; and maternal love or
maternal hatred, though the latter fortunately is most rare, is all the
same to the inexorable principle of natural selection. If we admire the
several ingenious contrivances, by which the flowers of the orchis and of
many other plants are fertilised through insect agency, can we consider as
equally perfect the elaboration by our fir-trees of dense clouds of pollen,
in order that a few granules may be wafted by a chance breeze on to the
Summary of Chapter -- We have in this chapter discussed some of the
difficulties and objections which may be urged against my theory. Many of
them are very grave; but I think that in the discussion light has been
thrown on several facts, which on the theory of independent acts of
creation are utterly obscure. We have seen that species at any one period
are not indefinitely variable
, and are not linked together by a multitude
of intermediate gradations, partly because the process of natural selection
will always be very slow, and will act, at any one time, only on a very few
forms; and partly because the very process of natural selection almost
implies the continual supplanting and extinction of preceding and
intermediate gradations. Closely allied species, now living on a
continuous area, must often have been formed when the area was not
continuous, and when the conditions of life did not insensibly graduate
away from one part to another. When two varieties are formed in two
districts of a continuous area, an intermediate variety will often be
formed, fitted for an intermediate zone; but from reasons assigned, the
intermediate variety will usually exist in lesser numbers than the two
forms which it connects; consequently the two latter, during the course of
further modification, from existing in greater numbers, will have a great
advantage over the less numerous intermediate variety, and will thus
generally succeed in supplanting and exterminating it.
We have seen in this chapter how cautious we should be in concluding that
the most different habits of life could not graduate into each other; that
a bat, for instance, could not have been formed by natural selection from
an animal which at first could only glide through the air.
We have seen that a species may under new conditions of life change its
habits, or have diversified habits, with some habits very unlike those of
its nearest congeners. Hence we can understand, bearing in mind that each
organic being is trying to live wherever it can live, how it has arisen
that there are upland geese with webbed feet, ground woodpeckers, diving
thrushes, and petrels with the habits of auks.
Although the belief that an organ so perfect as the eye could have been
formed by natural selection, is more than enough to stagger any one; yet in
the case of any organ, if we know of a long series of gradations in
complexity, each good for its possessor, then, under changing conditions of
life, there is no logical impossibility in the acquirement of any
conceivable degree of perfection through natural selection. In the cases
in which we know of no intermediate or transitional states, we should be
very cautious in concluding that none could have existed, for the
homologies of many organs and their intermediate states show that wonderful
metamorphoses in function are at least possible. For instance, a
swim-bladder has apparently been converted into an air-breathing lung. The
same organ having performed simultaneously very different functions, and
then having been specialised for one function; and two very distinct organs
having performed at the same time the same function, the one having been
perfected whilst aided by the other, must often have largely facilitated
transitions.
We are far too ignorant, in almost every case, to be enabled to assert that
any part or organ is so unimportant for the welfare of a species, that
modifications in its structure could not have been slowly accumulated by
means of natural selection. But we may confidently believe that many
modifications, wholly due to the laws of growth, and at first in no way
advantageous to a species, have been subsequently taken advantage of by the
still further modified descendants of this species. We may, also, believe
that a part formerly of high importance has often been retained (as the
tail of an aquatic animal by its terrestrial descendants), though it has
become of such small importance that it could not, in its present state,
have been acquired by natural selection,--a power which acts solely by the
preservation of profitable variations in the struggle for life.
Natural selection will produce nothing in one species for the exclusive