and plants with separated sexes, it is of course obvious that
two individuals must always (with the exception of the curious
and not well understood cases of parthenogenesis) unite for
each birth; but in the case of hermaphrodites this is far from
obvious. Nevertheless there is reason to believe that with all
hermaphrodites two individuals, either occasionally or habitually,
concur for the reproduction of their kind. This view was long ago
doubtfully suggested by Sprengel, Knight, and Kölreuter. We shall
presently see its importance; but I must here treat the subject
with extreme brevity, though I have the materials prepared for an
ample discussion. All vertebrate animals, all insects, and some
other large groups of animals, pair for each birth. Modern research
has much diminished the number of supposed hermaphrodites; and of
real hermaphrodites a large number pair; that is, two individuals
regularly unite for reproduction, which is all that concerns us. But
still there are many hermaphrodite animals which certainly do not
habitually pair, and a vast majority of plants are hermaphrodites.
What reason, it may be asked, is there for supposing in these cases
that two individuals ever concur in reproduction? As it is impossible
here to enter on details, I must trust to some general considerations
alone.
In the first place, I have collected so large a body of facts, and
made so many experiments, showing, in accordance with the almost
universal belief of breeders, that with animals and plants a cross
between different varieties, or between individuals of the same
variety but of another strain, gives vigor and fertility to the
offspring; and, on the other hand, that _close_ interbreeding
diminishes vigor and fertility; that these facts alone incline me
to believe that it is a general law of nature that no organic being
fertilizes itself for a perpetuity of generations; but that a cross
with another individual is occasionally--perhaps at long intervals of
time--indispensable.
Turning for a brief space to animals; various terrestrial species are
hermaphrodites, such as the land-mollusca and earthworms; but these
all pair. As yet I have not found a single terrestrial animal which
can fertilize itself. This remarkable fact, which offers so strong
a contrast with terrestrial plants, is intelligible on the view of
an occasional cross being indispensable; for owing to the nature of
the fertilizing element there are no means, analogous to the action
of insects and of the wind with plants, by which an occasional cross
could be effected with terrestrial animals without the concurrence of
two individuals. Of aquatic animals, there are many self-fertilizing
hermaphrodites; but here the currents of water offer an obvious
means for an occasional cross. As in the case of flowers, I have as
yet failed, after consultation with one of the highest authorities,
namely, Professor Huxley, to discover a single hermaphrodite animal
with the organs of reproduction so perfectly inclosed that access
from without, and the occasional influence of a distinct individual,
can be shown to be physically impossible. Cirripeds long appeared to
me to present, under this point of view, a case of great difficulty;
but I have been enabled, by a fortunate chance, to prove that two
individuals, though both are self-fertilizing hermaphrodites, do
sometimes cross.
It must have struck most naturalists as a strange anomaly that,
both with animals and plants, some species of the same family and
even of the same genus, though agreeing closely with each other in
their whole organization, are hermaphrodites, and some unisexual.
But if, in fact, all hermaphrodites do occasionally intercross, the
difference between them and unisexual species is, as far as function
is concerned, very small.
From these several considerations and from the many special facts
which I have collected, but which I am unable here to give, it
appears that with animals and plants an occasional intercross between
distinct individuals is a very general, if not universal, law of
nature.
To sum up, as far as the extreme intricacy of the subject permits,
the circumstances favorable and unfavorable for the production of new
species through natural selection. I conclude that for terrestrial
productions a large continental area, which has undergone many
oscillations of level, will have been the most favorable for the
production of any new forms of life, fitted to endure for a long time
and to spread widely. While the area existed as a continent, the
inhabitants will have been numerous in individuals and kinds, and
will have been subjected to severe competition. When converted by
subsidence into large separate islands, there will still have existed
many individuals of the same species on each island: intercrossing
on the confines of the range of each new species will have been
checked: after physical changes of any kind, immigration will have
been prevented, so that new places in the polity of each island will
have had to be filled up by the modification of the old inhabitants;
and time will have been allowed for the varieties in each to become
well modified and perfected. When, by renewed elevation, the islands
were reconverted into a continental area, there will again have been
very severe competition: the most favored or improved varieties will
have been enabled to spread: there will have been much extinction of
the less improved forms, and the relative proportional numbers of
the various inhabitants of the reunited continent will again have
been changed; and again there will have been a fair field for natural
selection to improve still further the inhabitants, and thus to
produce new species.
That natural selection generally acts with extreme slowness I fully
admit. It can act only when there are places in the natural polity
of a district which can be better occupied by the modification of
some of its existing inhabitants. The occurrence of such places
will often depend on physical changes, which generally take place
very slowly, and on the immigration of better adapted forms being
prevented. As some few of the old inhabitants become modified, the
mutual relations of others will often be disturbed; and this will
create new places, ready to be filled up by better adapted forms; but
all this will take place very slowly. Although all the individuals
of the same species differ in some slight degree from each other,
it would often be long before differences of the right nature in
various parts of the organization might occur. The result would often
be greatly retarded by free intercrossing. Many will exclaim that
these several causes are amply sufficient to neutralize the power of
natural selection. I do not believe so. But I do believe that natural
selection will generally act very slowly, only at long intervals of
time, and only on a few of the inhabitants of the same region. I
further believe that these slow, intermittent results accord well
with what geology tells us of the rate and manner at which the
inhabitants of the world have changed.
Slow though the process of selection may be, if feeble man can do
much by artificial selection, I can see no limit to the amount of
change, to the beauty and complexity of the co-adaptations between
all organic beings, one with another and with their physical
conditions of life, which may have been effected in the long course
of time through nature’s power of selection, that is, by the survival
of the fittest.
Natural selection acts solely through the preservation of variations
in some way advantageous, which consequently endure. Owing to the
high geometrical rate of increase of all organic beings, each area
is already fully stocked with inhabitants; and it follows from this,
that as the favored forms increase in number, so, generally, will
the less favored decrease and become rare. Rarity, as geology tells
us, is the precursor to extinction. We can see that any form which
is represented by few individuals will run a good chance of utter
extinction, during great fluctuations in the nature of the seasons,
or from a temporary increase in the number of its enemies. But we may
go further than this; for, as new forms are produced, unless we admit
that specific forms can go on indefinitely increasing in number, many
old forms must become extinct. That the number of specific forms has
not indefinitely increased, geology plainly tells us; and we shall
attempt to show why it is that the number of species throughout the
world has not become immeasurably great.
We have seen that the species which are most numerous in individuals
have the best chance of producing favorable variations within any
given period. It is the common and diffused or dominant species which
offer the greatest number of recorded varieties. Hence, rare species
will be less quickly modified or improved within any given period;
they will consequently be beaten in the race for life by the modified
and improved descendants of the commoner species.
From these several considerations I think it inevitably follows that
as new species in the course of time are formed through natural
selection, others will become rarer and rarer, and finally extinct.
The forms which stand in closest competition with those undergoing
modification and improvement will naturally suffer most. And we have
seen in the chapter on the Struggle for Existence that it is the most
closely allied forms--varieties of the same species, and species of
the same genus or of related genera--which, from having nearly the
same structure, constitution, and habits, generally come into the
severest competition with each other; consequently, each new variety
or species, during the progress of its formation, will generally
press hardest on its nearest kindred, and tend to exterminate them.
We see the same process of extermination among our domesticated
productions, through the selection of improved forms by man. Many
curious instances could be given showing how quickly new breeds of
cattle, sheep, and other animals, and varieties of flowers, take the
place of older and inferior kinds. In Yorkshire, it is historically
known that the ancient black cattle were displaced by the long-horns,
and that these “were swept away by the short-horns” (I quote the
words of an agricultural writer) “as if by some murderous pestilence.”
Divergence of character is of high importance, and explains, as I
believe, several important facts. In the first place, varieties,
even strongly marked ones, though having somewhat of the character
of species--as is shown by the hopeless doubts in many cases of how
to rank them--yet certainly differ far less from each other than
do good and distinct species. Nevertheless, 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.
We can clearly discern this in the case of animals with simple
habits. Take the case of a carnivorous quadruped, of which the number
that can be supported in any country has long ago arrived at its
full average. If its natural power of increase be allowed to act,
it can succeed in increasing (the country not undergoing any change
in conditions) only by its varying descendants seizing on places at
present occupied by other animals: some of them, for instance, being
enabled to feed on new kinds of prey, either dead or alive; some
inhabiting new stations, climbing trees, frequenting water, and some
perhaps becoming less carnivorous. The more diversified in habits and
structure the descendants of our carnivorous animals become, the more
places they will be enabled to occupy. What applies to one animal
will apply throughout all time to all animals--that is, if they
vary--for otherwise natural selection can effect nothing.
By considering the nature of the plants or animals which have in
any country struggled successfully with the indigenes, and have
there become naturalized, we may gain some crude idea in what manner
some of the natives would have to be modified in order to gain an
advantage over their compatriots; and we may at least infer that
diversification of structure, amounting to new generic differences,
would be profitable to them.
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.
If under changing conditions of life organic beings present
individual differences in almost every part of their structure, and
this can not be disputed; if there be, owing to their geometrical
rate of increase, a severe struggle for life at some age, season, or
year, and this certainly can not be disputed; then, considering the
infinite complexity of the relations of all organic beings to each
other and to their conditions of life, causing an infinite diversity
in structure, constitution, and habits, to be advantageous to them,
it would be a most extraordinary fact if no variations had ever
occurred useful to each being’s own welfare, in the same manner as
so many variations have occurred useful to man. But if variations
useful to any organic being ever do occur, assuredly individuals
thus characterized will have the best chance of being preserved in
the struggle for life; and from the strong principle of inheritance,
these will tend to produce offspring similarly characterized. This
principle of preservation, or the survival of the fittest, I have
called Natural Selection. It leads to the improvement of each
creature in relation to its organic and inorganic conditions of life;
and consequently, in most cases, to what must be regarded as an
advance in organization. Nevertheless, low and simple forms will long
endure if well fitted for their simple conditions of life.
Natural selection, on the principle of qualities being inherited at
corresponding ages, can modify the egg, seed, or young, as easily as
the adult. Among many animals, sexual selection will have given its
aid to ordinary selection, by assuring to the most vigorous and best
adapted males the greatest number of offspring. Sexual selection will
also give characters useful to the males alone, in their struggles or
rivalry with other males; and these characters will be transmitted to
one sex or to both sexes, according to the form of inheritance which
prevails.
Whether natural selection has really thus acted, in adapting the
various forms of life to their several conditions and stations, must
be judged by the general tenor and balance of evidence. But we have
already seen how it entails extinction; and how largely extinction
has acted in the world’s history, geology plainly declares. Natural
selection, also, leads to divergence of character; for the more
organic beings diverge in structure, habits, and constitution, by so
much the more can a large number be supported on the area--of which
we see proof by looking to the inhabitants of any small spot, and
to the productions naturalized in foreign lands. Therefore, during
the modification of the descendants of any one species, and during
the incessant struggle of all species to increase in numbers, the
more diversified the descendants become, the better will be their
chance of success in the battle for life. Thus the small differences
distinguishing varieties of the same species steadily tend to
increase, till they equal the greater differences between species of
the same genus, or even of distinct genera.
We have seen that it is the common, the widely diffused and widely
ranging species, belonging to the larger genera within each class,
which vary most; and these tend to transmit to their modified
offspring that superiority which now makes them dominant in their own
countries. Natural selection, as has just been remarked, leads to
divergence of character and to much extinction of the less improved
and intermediate forms of life. On these principles, the nature of
the affinities, and the generally well-defined distinctions between
the innumerable organic beings in each class throughout the world,
may be explained. It is a truly wonderful fact--the wonder of which
we are apt to overlook from familiarity--that all animals and all
plants throughout all time and space should be related to each other
in groups, subordinate to groups, in the manner which we everywhere
behold; namely, varieties of the same species most closely related,
species of the same genus less closely and unequally related,
forming sections and sub-genera, species of distinct genera much
less closely related, and genera related in different degrees,
forming sub-families, families, orders, sub-classes and classes.
The several subordinate groups in any class can not be ranked in a
single file, but seem clustered round points, and these round other
points, and so on in almost endless cycles. If species had been
independently created, no explanation would have been possible of
this kind of classification; but it is explained through inheritance
and the complex action of natural selection, entailing extinction and
divergence of character.
The affinities of all the beings of the same class have sometimes
been represented by a great tree. I believe this simile largely
speaks the truth. The green and budding twigs may represent existing
species; and those produced during former years may represent the
long succession of extinct species. At each period of growth all the
growing twigs have tried to branch out on all sides, and to overtop
and kill the surrounding twigs and branches, in the same manner
as species and groups of species have at all times overmastered
other species in the great battle for life. The limbs divided into
great branches, and these into lesser and lesser branches, were
themselves once, when the tree was young, budding twigs; and this
connection of the former and present buds by ramifying branches
may well represent the classification of all extinct and living
species in groups subordinate to groups. Of the many twigs which
flourished when the tree was a mere bush, only two or three, now
grown into great branches, yet survive and bear the other branches;
so with the species which lived during long-past geological periods,
very few have left living and modified descendants. From the first
growth of the tree, many a limb and branch has decayed and dropped
off; and these fallen branches of various sizes may represent
those whole orders, families, and genera which have now no living
representatives, and which are known to us only in a fossil state.
As we here and there see a thin straggling branch springing from a
fork low down in a tree, and which by some chance has been favored
and is still alive on its summit, so we occasionally see an animal
like the Ornithorhynchus or Lepidosiren, which in some small degree
connects by its affinities two large branches of life, and which has
apparently been saved from fatal competition by having inhabited
a protected station. As buds give rise by growth to fresh buds,
and these, if vigorous, branch out and overtop on all sides many a
feebler branch, so by generation I believe it has been with the great
Tree of Life, which fills with its dead and broken branches the crust
of the earth, and covers the surface with its ever-branching and
beautiful ramifications.
MAMMALIA
--BARON CUVIER
The mammalia are placed at the head of the animal kingdom not only
because it is the class to which man himself belongs, but also
because it is that which enjoys the most numerous faculties, the most
delicate sensations, the most varied powers of motion, and in which
all the different qualities seem combined in order to produce a most
perfect degree of intelligence, the one most fertile in resources,
most susceptible of perfection, and least the slave of instinct.