species exclusively for the good of another species; though throughout
nature one species incessantly takes advantage of, and profits by, the
structures of others. But natural selection can and does often produce
structures for the direct injury of other animals, 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 can not find even one which seems to me of
any weight. It is admitted that the rattlesnake has a poison-fang for
its own defense, and for the destruction of its prey; but some authors
suppose that at the same time it is furnished with a rattle for its
own injury, namely, to warn its prey. 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. It is a much more probable view that
the rattlesnake uses its rattle, the cobra expands its frill, and the
puff-adder swells while hissing so loudly and harshly, in order to
alarm the many birds and beasts which are known to attack even the
most venomous species. Snakes act on the same principle which makes
the hen ruffle her feathers and expand her wings when a dog approaches
her chickens; but I have not space here to enlarge on the many ways by
which animals endeavor to frighten away their enemies.
Natural selection will never produce in a being any structure more
injurious than beneficial to that being, 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 comes into competition. And we see that this
is the standard 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 by Müller not to be perfect even in
that most perfect organ, the human eye.
* * * * *
[Page 67.]
Natural selection will modify the structure of the young in relation
to the parent, and of the parent in relation to the young. In social
animals it will adapt the structure of each individual for the benefit
of the whole community, if the community profits by the selected
change. What natural selection can not do is, to modify the structure
of one species, without giving it any advantage, for the good of
another species; and, though statements to this effect may be found
in works of natural history, I can not find one case which will bear
investigation. A structure used only once in an animal’s life, if of
high importance to it, might be modified to any extent by natural
selection; for instance, the great jaws possessed by certain insects,
used exclusively for opening the cocoon, or the hard tip to the beak of
unhatched birds, used for breaking the egg. It has been asserted that,
of the best short-beaked tumbler-pigeons, a greater number perish in
the egg than are able to get out of it, so that fanciers assist in the
act of hatching. Now, if Nature had to make the beak of a full-grown
pigeon very short for the bird’s own advantage, the process of
modification would be very slow, and there would be simultaneously the
most rigorous selection of all the young birds within the egg, which
had the most powerful and hardest beaks, for all with weak beaks would
inevitably perish; or, more delicate and more easily broken shells
might be selected, the thickness of the shell being known to vary like
every other structure.
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION.
[Origin of
Species,
page 70.]
In order to make it clear how, as I believe, natural selection acts, I
must beg permission to give one or two imaginary illustrations. Let us
take the case of a wolf, which preys on various animals, securing some
by craft, some by strength, and some by fleetness; and let us suppose
that the fleetest prey, a deer for instance, had from any change in
the country increased in numbers, or that other prey had decreased
in numbers, during that season of the year when the wolf was hardest
pressed for food. Under such circumstances the swiftest and slimmest
wolves would have the best chance of surviving, and so be preserved or
selected--provided always that they retained strength to master their
prey at this or some other period of the year, when they were compelled
to prey on other animals. I can see no more reason to doubt that this
would be the result, than that man should be able to improve the
fleetness of his greyhounds by careful and methodical selection, or by
that kind of unconscious selection which follows from each man trying
to keep the best dogs without any thought of modifying the breed. I
may add that, according to Mr. Pierce, there are two varieties of the
wolf inhabiting the Catskill Mountains in the United States, one with
a light greyhound-like form, which pursues deer, and the other more
bulky, with shorter legs, which more frequently attacks the shepherd’s
flocks.
* * * * *
[Page 73.]
Certain plants excrete sweet juice, apparently for the sake of
eliminating something injurious from the sap: this is effected, for
instance, by glands at the base of the stipules in some _Leguminosæ_,
and at the backs of the leaves of the common laurel. This juice, though
small in quantity, is greedily sought by insects; but their visits do
not in any way benefit the plant. Now, let us suppose that the juice
or nectar was excreted from the inside of the flowers of a certain
number of plants of any species. Insects in seeking the nectar would
get dusted with pollen, and would often transport it from one flower to
another. The flowers of two distinct individuals of the same species
would thus get crossed; and the act of crossing, as can be fully
proved, gives rise to vigorous seedlings, which consequently would
have the best chance of flourishing and surviving. The plants which
produced flowers with the largest glands or nectaries, excreting most
nectar, would oftenest be visited by insects, and would oftenest be
crossed; and so in the long run would gain the upper hand and form a
local variety. The flowers, also, which had their stamens and pistils
placed, in relation to the size and habits of the particular insect
which visited them, so as to favor in any degree the transportal of
the pollen, would likewise be favored. We might have taken the case of
insects visiting flowers for the sake of collecting pollen instead of
nectar; and, as pollen is formed for the sole purpose of fertilization,
its destruction appears to be a simple loss to the plant; yet if a
little pollen were carried, at first occasionally and then habitually,
by the pollen-devouring insects from flower to flower, and a cross
thus effected, although nine tenths of the pollen were destroyed,
it might still be a great gain to the plant to be thus robbed; and
the individuals which produced more and more pollen, and had larger
anthers, would be selected.
When our plant, by the above process long continued, had been rendered
highly attractive to insects, they would, unintentionally on their
part, regularly carry pollen from flower to flower.
DIVERGENCE OF CHARACTER.
[Page 86.]
According to my view, varieties are species in the process of
formation, or are, as I have called them, incipient species. How, then,
does the lesser difference between varieties become augmented into the
greater difference between species? That this does habitually happen,
we must infer from most of the innumerable species throughout nature
presenting well-marked differences; whereas varieties, the supposed
prototypes and parents of future well-marked species, present slight
and ill-defined differences. Mere chance, as we may call it, might
cause one variety to differ in some character from its parents, and
the offspring of this variety again to differ from its parent in the
very same character and in a greater degree; but this alone would
never account for so habitual and large a degree of difference as that
between the species of the same genus.
As has always been my practice, I have sought light on this head from
our domestic productions. We shall here find something analogous.
It will be admitted that the production of races so different as
short-horn and Hereford cattle, race and cart horses, the several
breeds of pigeons, etc., could never have been effected by the mere
chance accumulation of similar variations during many successive
generations. In practice, a fancier is, for instance, struck by a
pigeon having a slightly shorter beak; another fancier is struck by a
pigeon having a rather longer beak; and, on the acknowledged principle
that “fanciers do not and will not admire a medium standard, but
like extremes,” they both go on (as has actually occurred with the
sub-breeds of the tumbler-pigeon) choosing and breeding from birds
with longer and longer beaks, or with shorter and shorter beaks.
Again, we may suppose that, at an early period of history, the men
of one nation or district required swifter horses, while those of
another required stronger and bulkier horses. The early differences
would be very slight; but, in the course of time, from the continued
selection of swifter horses in the one case, and of stronger ones in
the other, the differences would become greater, and would be noted
as forming two sub-breeds. Ultimately, after the lapse of centuries,
these sub-breeds would become converted into two well-established
and distinct breeds. As the differences became greater, the inferior
animals with intermediate characters, being neither very swift nor
very strong, would not have been used for breeding, and will thus have
tended to disappear. Here, then, we see in man’s productions the action
of what may be called the principle of divergence, causing differences,
at first barely appreciable, steadily to increase, and the breeds to
diverge in character, both from each other and from their common parent.
But how, it may be asked, can any analogous principle apply in nature?
I believe it can and does apply most efficiently (though it was a long
time before I saw how), 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.
* * * * *
[Page 89.]
The advantage of diversification of structure in the inhabitants of the
same region is, in fact, the same as that of the physiological division
of labor in the organs of the same individual body--a subject so well
elucidated by Milne-Edwards. No physiologist doubts that a stomach
adapted to digest vegetable matter alone, or flesh alone, draws most
nutriment from these substances. So in the general economy of any land,
the more widely and perfectly the animals and plants are diversified
for different habits of life, so will a greater number of individuals
be capable of there supporting themselves. A set of animals, with
their organization but little diversified, could hardly compete with
a set more perfectly diversified in structure. It may be doubted, for
instance, whether the Australian marsupials, which are divided into
groups differing but little from each other, and feebly representing,
as Mr. Waterhouse and others have remarked, our carnivorous,
ruminant, and rodent mammals, could successfully compete with these
well-developed orders. In the Australian mammals, we see the process of
diversification in an early and incomplete stage of development.
EVOLUTION OF THE HUMAN EYE.
[Origin of
Species,
page 143.]
To suppose that the eye with all its inimitable contrivances for
adjusting the focus to different distances, for admitting different
amounts of light, and for the correction of spherical and chromatic
aberration, could have been formed by natural selection, seems, I
freely confess, absurd in the highest degree. When it was first said
that the sun stood still and the world turned round, the common sense
of mankind declared the doctrine false; but the old saying of _Vox
populi vox Dei_, as every philosopher knows, can not be trusted in
science.
* * * * *
[Page 145.]
Within the highest division of the animal kingdom, namely, the
_Vertebrata_, we can start from an eye so simple that it consists, as
in the lancelet, of a little sac of transparent skin, furnished with
a nerve and lined with pigment, but destitute of any other apparatus.
In fishes and reptiles, as Owen has remarked, “the range of gradations
of dioptric structures is very great.” It is a significant fact that
even in man, according to the high authority of Virchow, the beautiful
crystalline lens is formed in the embryo by an accumulation of
epidermic cells, lying in a sac-like fold of the skin; and the vitreous
body is formed from embryonic subcutaneous tissue. To arrive, however,
at a just conclusion regarding the formation of the eye, with all its
marvelous yet not absolutely perfect characters, it is indispensable
that the reason should conquer the imagination; but I have felt the
difficulty far too keenly to be surprised at others hesitating to
extend the principle of natural selection to so startling a length.
It is scarcely possible to avoid comparing the eye with a telescope.
We know that this instrument has been perfected by the long-continued
efforts of the highest human intellects; and we naturally infer that
the eye has been formed by a somewhat analogous process. But may not
this inference be presumptuous? Have we any right to assume that the
Creator works by intellectual powers like those of man? If we must
compare the eye to an optical instrument, we ought in imagination to
take a thick layer of transparent tissue, with spaces filled with
fluid, and with a nerve sensitive to light beneath, and then suppose
every part of this layer to be continually changing slowly in density,
so as to separate into layers of different densities and thicknesses,
placed at different distances from each other, and with the surfaces
of each layer slowly changing in form. Further, we must suppose that
there is a power, represented by natural selection or the survival
of the fittest, always intently watching each slight alteration in
the transparent layers; and carefully preserving each which, under
varied circumstances, in any way or in any degree, tends to produce
a distincter image. We must suppose each new state of the instrument
to be multiplied by the million; each to be preserved until a better
one is produced, and then the old ones to be all destroyed. In living
bodies, variation will cause the slight alterations, generation will
multiply them almost infinitely, and natural selection will pick out
with unerring skill each improvement. Let this process go on for
millions of years; and during each year on millions of individuals of
many kinds; and may we not believe that a living optical instrument
might thus be formed as superior to one of glass as the works of the
Creator are to those of man?
VI.
GEOGRAPHICAL DISTRIBUTION OF ORGANIC BEINGS.
[Origin of
Species,
page 320.]
We are thus brought to the question which has been largely discussed
by naturalists, namely, whether species have been created at one or
more points of the earth’s surface. Undoubtedly there are many cases of
extreme difficulty in understanding how the same species could possibly
have migrated from some one point to the several distant and isolated
points where now found. Nevertheless the simplicity of the view that
each species was first produced within a single region captivates the
mind. He who rejects it rejects the _vera causa_ of ordinary generation
with subsequent migration, and calls in the agency of a miracle. It is
universally admitted that in most cases the area inhabited by a species
is continuous; and that, when a plant or animal inhabits two points so
distant from each other, or with an interval of such a nature, that the
space could not have been easily passed over by migration, the fact
is given as something remarkable and exceptional. The incapacity of
migrating across a wide sea is more clear in the case of terrestrial
mammals than perhaps with any other organic beings; and, accordingly,
we find no inexplicable instances of the same mammals inhabiting
distant points of the world. No geologist feels any difficulty in
Great Britain possessing the same quadrupeds with the rest of Europe,
for they were no doubt once united. But, if the same species can be
produced at two separate points, why do we not find a single mammal
common to Europe and Australia or South America? The conditions of
life are nearly the same, so that a multitude of European animals and
plants have become naturalized in America and Australia; and some of
the aboriginal plants are identically the same at these distant points
of the northern and southern hemispheres. The answer, as I believe,
is, that mammals have not been able to migrate, whereas some plants,
from their varied means of dispersal, have migrated across the wide
and broken interspaces. The great and striking influence of barriers
of all kinds is intelligible only on the view that the great majority
of species have been produced on one side, and have not been able to
migrate to the opposite side. Some few families, many sub-families,
very many genera, and a still greater number of sections of genera,
are confined to a single region: and it has been observed by several
naturalists that the most natural genera, or those genera in which the
species are most closely related to each other, are generally confined
to the same country, or, if they have a wide range, that their range is
continuous. What a strange anomaly it would be, if a directly opposite
rule were to prevail, when we go down one step lower in the series,
namely, to the individuals of the same species, and these had not been,
at least at first, confined to some one region!
Hence it seems to me, as it has to many other naturalists, that the
view of each species having been produced in one area alone, and
having subsequently migrated from that area as far as its powers of
migration and subsistence under past and present conditions permitted,
is the most probable. Undoubtedly many cases occur, in which we can
not explain how the same species could have passed from one point
to the other. But the geographical and climatal changes, which have
certainly occurred within recent geological times, must have rendered
discontinuous the formerly continuous range of many species. So that we
are reduced to consider whether the exceptions to continuity of range
are so numerous and of so grave a nature that we ought to give up the
belief, rendered probable by general considerations, that each species
has been produced within one area, and has migrated thence as far as it
could.
ISOLATED CONTINENTS NEVER WERE UNITED.
[Origin of
Species,
page 324.]
Whenever it is fully admitted, as it will some day be, that each
species has proceeded from a single birthplace, and when in the course
of time we know something definite about the means of distribution, we
shall be enabled to speculate with security on the former extension
of the land. But I do not believe that it will ever be proved that
within the recent period most of our continents which now stand quite
separate have been continuously, or almost continuously, united with
each other, and with the many existing oceanic islands. Several facts
in distribution, such as the great difference in the marine faunas on
the opposite sides of almost every continent, the close relation of the
tertiary inhabitants of several lands and even seas to their present
inhabitants, the degree of affinity between the mammals inhabiting
islands with those of the nearest continent, being in part determined
(as we shall hereafter see) by the depth of the intervening ocean,
these and other such facts are opposed to the admission of such
prodigious geographical revolutions within the recent period as are
necessary on the view advanced by Forbes and admitted by his followers.
The nature and relative proportions of the inhabitants of oceanic
islands are likewise opposed to the belief of their former continuity
with continents. Nor does the almost universally volcanic composition
of such islands favor the admission that they are the wrecks of
sunken continents; if they had originally existed as continental
mountain-ranges, some at least of the islands would have been formed,
like other mountain-summits, of granite, metamorphic schists, old
fossiliferous and other rocks, instead of consisting of mere piles of
volcanic matter.
MEANS OF DISPERSAL.
[Page 326.]
Living birds can hardly fail to be highly effective agents in the
transportation of seeds. I could give many facts showing how frequently
birds of many kinds are blown by gales to vast distances across the
ocean. We may safely assume that under such circumstances their rate
of flight would often be thirty-five miles an hour; and some authors
have given a far higher estimate. I have never seen an instance of
nutritious seeds passing through the intestines of a bird; but hard
seeds of fruit pass uninjured through even the digestive organs of a
turkey. In the course of two months I picked up in my garden twelve
kinds of seeds out of the excrement of small birds, and these seemed
perfect, and some of them, which were tried, germinated. But the
following fact is more important: the crops of birds do not secrete
gastric juice, and do not, as I know by trial, injure in the least the
germination of seeds; now, after a bird has found and devoured a large
supply of food, it is positively asserted that all the grains do not
pass into the gizzard for twelve or even eighteen hours. A bird in this
interval might easily be blown to the distance of five hundred miles,
and hawks are known to look out for tired birds, and the contents of
their torn crops might thus readily get scattered. Some hawks and owls
bolt their prey whole, and, after an interval of from twelve to twenty
hours, disgorge pellets, which, as I know from experiments made in the
Zoölogical Gardens, include seeds capable of germination. Some seeds
of the oat, wheat, millet, canary, hemp, clover, and beet germinated
after having been from twelve to twenty-one hours in the stomachs of
different birds of prey; and two seeds of beet grew after having been
thus retained for two days and fourteen hours. Fresh-water fish, I
find, eat seeds of many land and water plants: fish are frequently
devoured by birds, and thus the seeds might be transported from place
to place. I forced many kinds of seeds into the stomachs of dead fish,
and then gave their bodies to fishing-eagles, storks, and pelicans;
these birds, after an interval of many hours, either rejected the seeds
in pellets or passed them in their excrement; and several of these
seeds retained the power of germination. Certain seeds, however, were
always killed by this process.
Locusts are sometimes blown to great distances from the land; I myself
caught one three hundred and seventy miles from the coast of Africa,
and have heard of others caught at greater distances.
* * * * *
[Page 328.]