 Chapter
4 - Natural Selection
*
Natural Selection *
its power compared with man's selection *
its power on characters of trifling importance *
its power at all ages and on both sexes *
Sexual Selection *
On the generality of intercrosses between individuals of
the same species *
Circumstances favourable and unfavourable to Natural Selection,
namely, intercrossing, isolation, number of individuals
*
Slow action *
Extinction caused by Natural Selection *
Divergence of Character, related to the diversity of inhabitants
of any small area, and to naturalisation *
Action of Natural Selection, through Divergence of Character
and Extinction, on the descendants from a common parent
*
Explains the Grouping of all organic beings
How will the struggle for existence, discussed too briefly
in the last chapter, act in regard to variation? Can the
principle of selection, which we have seen is so potent
in the hands of man, apply in nature? I think we shall see
that it can act most effectually. Let it be borne in mind
in what an endless number of strange peculiarities our domestic
productions, and, in a lesser degree, those under nature,
vary; and how strong the hereditary tendency is. Under domestication,
it may be truly said that the, whole organisation becomes
in some degree plastic. Let it be borne in mind how infinitely
complex and close-fitting are the mutual relations of all
organic beings to each other and to their physical conditions
of life. Can it, then, be thought improbable, seeing that
variations useful to man have undoubtedly occurred, that
other variations useful in some way to each being in the
great and complex battle of life, should sometimes occur
in the course of thousands of generations? If such do occur,
can we doubt (remembering that many more individuals are
born than can possibly survive) that individuals having
any advantage, however slight, over others, would have the
best chance of surviving and of procreating their kind?
On the other hand, we may feel sure that any variation in
the least degree injurious would be rigidly destroyed. This
preservation of favourable variations and the rejection
of injurious variations, I call Natural Selection. Variations
neither useful nor injurious would not be affected by natural
selection, and would be left a fluctuating element, as perhaps
we see in the species called polymorphic.
We shall best understand the probable course of natural
selection by taking the case of a country undergoing some
physical change, for instance, of climate. The proportional
numbers of its inhabitants would almost immediately undergo
a change, and some species might become extinct. We may
conclude, from what we have seen of the intimate and complex
manner in which the inhabitants of each country are bound
together, that any change in the numerical proportions of
some of the inhabitants, independently of the change of
climate itself, would most seriously affect many of the
others. If the country were open on its borders, new forms
would certainly immigrate, and this also would seriously
disturb the relations of some of the former inhabitants.
Let it be remembered how powerful the influence of a single
introduced tree or mammal has been shown to be. But in the
case of an island, or of a country partly surrounded by
barriers, into which new and better adapted forms could
not freely enter, we should then have places in the economy
of nature which would assuredly be better filled up, if
some of the original inhabitants were in some manner modified;
for, had the area been open to immigration, these same places
would have been seized on by intruders. In such case, every
slight modification, which in the course of ages chanced
to arise, and which in any way favoured the individuals
of any of the species, by better adapting them to their
altered conditions, would tend to be preserved; and natural
selection would thus have free scope for the work of improvement.
We have reason to believe, as stated in the first chapter,
that a change in the conditions of life, by specially acting
on the reproductive system, causes or increases variability;
and in the foregoing case the conditions of life are supposed
to have undergone a change, and this would manifestly be
favourable to natural selection, by giving a better chance
of profitable variations occurring; and unless profitable
variations do occur, natural selection can do nothing. Not
that, as I believe, any extreme amount of variability is
necessary; as man can certainly produce great results by
adding up in any given direction mere individual differences,
so could Nature, but far more easily, from having incomparably
longer time at her disposal. Nor do I believe that any great
physical change, as of climate, or any unusual degree of
isolation to check immigration, is actually necessary to
produce new and unoccupied places for natural selection
to fill up by modifying and improving some of the varying
inhabitants. For as all the inhabitants of each country
are struggling together with nicely balanced forces, extremely
slight modifications in the structure or habits of one inhabitant
would often give it an advantage over others; and still
further modifications of the same kind would often still
further increase the advantage. No country can be named
in which all the native inhabitants are now so perfectly
adapted to each other and to the physical conditions under
which they live, that none of them could anyhow be improved;
for in all countries, the natives have been so far conquered
by naturalised productions, that they have allowed foreigners
to take firm possession of the land. And as foreigners have
thus everywhere beaten some of the natives, we may safely
conclude that the natives might have been modified with
advantage, so as to have better resisted such intruders.
As man can produce and certainly has produced a great
result by his methodical and unconscious means of selection,
what may not nature effect? Man can act only on external
and visible characters: nature cares nothing for appearances,
except in so far as they may be useful to any being. She
can act on every internal organ, on every shade of constitutional
difference, on the whole machinery of life. Man selects
only for his own good; Nature only for that of the being
which she tends. Every selected character is fully exercised
by her; and the being is placed under well-suited conditions
of life. Man keeps the natives of many climates in the same
country; he seldom exercises each selected character in
some peculiar and fitting manner; he feeds a long and a
short beaked pigeon on the same food; he does not exercise
a long-backed or long-legged quadruped in any peculiar manner;
he exposes sheep with long and short wool to the same climate.
He does not allow the most vigorous males to struggle for
the females. He does not rigidly destroy all inferior animals,
but protects during each varying season, as far as lies
in his power, all his productions. He often begins his selection
by some half-monstrous form; or at least by some modification
prominent enough to catch his eye, or to be plainly useful
to him. Under nature, the slightest difference of structure
or constitution may well turn the nicely-balanced scale
in the struggle for life, and so be preserved. How fleeting
are the wishes and efforts of man! how short his time! and
consequently how poor will his products be, compared with
those accumulated by nature during whole geological periods.
Can we wonder, then, that nature's productions should be
far 'truer' in character than man's productions; that they
should be infinitely better adapted to the most complex
conditions of life, and should plainly bear the stamp of
far higher workmanship?
It may be said that natural selection is daily and hourly
scrutinising, throughout the world, every variation, even
the slightest; rejecting that which is bad, preserving and
adding up all that is good; silently and insensibly working,
whenever and wherever opportunity offers, at the improvement
of each organic being in relation to its organic and inorganic
conditions of life. We see nothing of these slow changes
in progress, until the hand of time has marked the long
lapses of ages, and then so imperfect is our view into long
past geological ages, that we only see that the forms of
life are now different from what they formerly were.
Although natural selection can act only through and for
the good of each being, yet characters and structures, which
we are apt to consider as of very trifling importance, may
thus be acted on. When we see leaf-eating insects green,
and bark-feeders mottled-grey; the alpine ptarmigan white
in winter, the red-grouse the colour of heather, and the
black-grouse that of peaty earth, we must believe that these
tints are of service to these birds and insects in preserving
them from danger. Grouse, if not destroyed at some period
of their lives, would increase in countless numbers; they
are known to suffer largely from birds of prey; and hawks
are guided by eyesight to their prey, so much so, that on
parts of the Continent persons are warned not to keep white
pigeons, as being the most liable to destruction. Hence
I can see no reason to doubt that natural selection might
be most effective in giving the proper colour to each kind
of grouse, and in keeping that colour, when once acquired,
true and constant. Nor ought we to think that the occasional
destruction of an animal of any particular colour would
produce little effect: we should remember how essential
it is in a flock of white sheep to destroy every lamb with
the faintest trace of black. In plants the down on the fruit
and the colour of the flesh are considered by botanists
as characters of the most trifling importance: yet we hear
from an excellent horticulturist, Downing, that in the United
States smooth-skinned fruits suffer far more from a beetle,
a curculio, than those with down; that purple plums suffer
far more from a certain disease than yellow plums; whereas
another disease attacks yellow-fleshed peaches far more
than those with other coloured flesh. If, with all the aids
of art, these slight differences make a great difference
in cultivating the several varieties, assuredly, in a state
of nature, where the trees would have to struggle with other
trees and with a host of enemies, such differences would
effectually settle which variety, whether a smooth or downy,
a yellow or purple fleshed fruit, should succeed.
In looking at many small points of difference between
species, which, as far as our ignorance permits us to judge,
seem to be quite unimportant, we must not forget that climate,
food, &c., probably produce some slight and direct effect.
It is, however, far more necessary to bear in mind that
there are many unknown laws of correlation of growth, which,
when one part of the organisation is modified through variation,
and the modifications are accumulated by natural selection
for the good of the being, will cause other modifications,
often of the most unexpected nature.
As we see that those variations which under domestication
appear at any particular period of life, tend to reappear
in the offspring at the same period; for instance, in the
seeds of the many varieties of our culinary and agricultural
plants; in the caterpillar and cocoon stages of the varieties
of the silkworm; in the eggs of poultry, and in the colour
of the down of their chickens; in the horns of our sheep
and cattle when nearly adult; so in a state of nature, natural
selection will be enabled to act on and modify organic beings
at any age, by the accumulation of profitable variations
at that age, and by their inheritance at a corresponding
age. If it profit a plant to have its seeds more and more
widely disseminated by the wind, I can see no greater difficulty
in this being effected through natural selection, than in
the cotton-planter increasing and improving by selection
the down in the pods on his cotton-trees. Natural selection
may modify and adapt the larva of an insect to a score of
contingencies, wholly different from those which concern
the mature insect. These modifications will no doubt affect,
through the laws of correlation, the structure of the adult;
and probably in the case of those insects which live only
for a few hours, and which never feed, a large part of their
structure is merely the correlated result of successive
changes in the structure of their larvae. So, conversely,
modifications in the adult will probably often affect the
structure of the larva; but in all cases natural selection
will ensure that modifications consequent on other modifications
at a different period of life, shall not be in the least
degree injurious: for if they became so, they would cause
the extinction of the species.
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 community; if
each in consequence profits by the selected change. What
natural selection cannot 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 cannot find
one case which will bear investigation. A structure used
only once in an animal's whole 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,
and used exclusively for opening the cocoon or the hard
tip to the beak of nestling birds, used for breaking the
egg. It has been asserted, that of the best short-beaked
tumbler-pigeons more 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 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.
Sexual Selection
Inasmuch as peculiarities often appear under domestication
in one sex and become hereditarily attached to that sex,
the same fact probably occurs under nature, and if so, natural
selection will be able to modify one sex in its functional
relations to the other sex, or in relation to wholly different
habits of life in the two sexes, as is sometimes the case
with insects. And this leads me to say a few words on what
I call Sexual Selection. This depends, not on a struggle
for existence, but on a struggle between the males for possession
of the females; the result is not death to the unsuccessful
competitor, but few or no offspring. Sexual selection is,
therefore, less rigorous than natural selection. Generally,
the most vigorous males, those which are best fitted for
their places in nature, will leave most progeny. But in
many cases, victory will depend not on general vigour, but
on having special weapons, confined to the male sex. A hornless
stag or spurless cock would have a poor chance of leaving
offspring. Sexual selection by always allowing the victor
to breed might surely give indomitable courage, length to
the spur, and strength to the wing to strike in the spurred
leg, as well as the brutal cock-fighter, who knows well
that he can improve his breed by careful selection of the
best cocks. How low in the scale of nature this law of battle
descends, I know not; male alligators have been described
as fighting, bellowing, and whirling round, like Indians
in a war-dance, for the possession of the females; male
salmons have been seen fighting all day long; male stag-beetles
often bear wounds from the huge mandibles of other males.
The war is, perhaps, severest between the males of polygamous
animals, and these seem oftenest provided with special weapons.
The males of carnivorous animals are already well armed;
though to them and to others, special means of defence may
be given through means of sexual selection, as the mane
to the lion, the shoulder-pad to the boar, and the hooked
jaw to the male salmon; for the shield may be as important
for victory, as the sword or spear.
Amongst birds, the contest is often of a more peaceful
character. All those who have attended to the subject, believe
that there is the severest rivalry between the males of
many species to attract by singing the females. The rock-thrush
of Guiana, birds of paradise, and some others, congregate;
and successive males display their gorgeous plumage and
perform strange antics before the females, which standing
by as spectators, at last choose the most attractive partner.
Those who have closely attended to birds in confinement
well know that they often take individual preferences and
dislikes: thus Sir R. Heron has described how one pied peacock
was eminently attractive to all his hen birds. It may appear
childish to attribute any effect to such apparently weak
means: I cannot here enter on the details necessary to support
this view; but if man can in a short time give elegant carriage
and beauty to his bantams, according to his standard of
beauty, I can see no good reason to doubt that female birds,
by selecting, during thousands of generations, the most
melodious or beautiful males, according to their standard
of beauty, might produce a marked effect. I strongly suspect
that some well-known laws with respect to the plumage of
male and female birds, in comparison with the plumage of
the young, can be explained on the view of plumage having
been chiefly modified by sexual selection, acting when the
birds have come to the breeding age or during the breeding
season; the modifications thus produced being inherited
at corresponding ages or seasons, either by the males alone,
or by the males and females; but I have not space here to
enter on this subject.
Thus it is, as I believe, that when the males and females
of any animal have the same general habits of life, but
differ in structure, colour, or ornament, such differences
have been mainly caused by sexual selection; that is, individual
males have had, in successive generations, some slight advantage
over other males, in their weapons, means of defence, or
charms; and have transmitted these advantages to their male
offspring. Yet, I would not wish to attribute all such sexual
differences to this agency: for we see peculiarities arising
and becoming attached to the male sex in our domestic animals
(as the wattle in male carriers, horn-like protuberances
in the cocks of certain fowls, &c.), which we cannot
believe to be either useful to the males in battle, or attractive
to the females. We see analogous cases under nature, for
instance, the tuft of hair on the breast of the turkey-cock,
which can hardly be either useful or ornamental to this
bird; indeed, had the tuft appeared under domestication,
it would have been called a monstrosity.
Illustrations of the action of Natural Selection
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 is
hardest pressed for food. I can under such circumstances
see no reason to doubt that 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 at some other period of
the year, when they might be compelled to prey on other
animals. I can see no more reason to doubt this, than that
man can improve the fleetness of his greyhounds by careful
and methodical selection, or by that unconscious selection
which results from each man trying to keep the best dogs
without any thought of modifying the breed.
Even without any change in the proportional numbers of
the animals on which our wolf preyed, a cub might be born
with an innate tendency to pursue certain kinds of prey.
Nor can this be thought very improbable; for we often observe
great differences in the natural tendencies of our domestic
animals; one cat, for instance, taking to catch rats, another
mice; one cat, according to Mr. St. John, bringing home
winged game, another hares or rabbits, and another hunting
on marshy ground and almost nightly catching woodcocks or
snipes. The tendency to catch rats rather than mice is known
to be inherited. Now, if any slight innate change of habit
or of structure benefited an individual wolf, it would have
the best chance of surviving and of leaving offspring. Some
of its young would probably inherit the same habits or structure,
and by the repetition of this process, a new variety might
be formed which would either supplant or coexist with the
parent-form of wolf. Or, again, the wolves inhabiting a
mountainous district, and those frequenting the lowlands,
would naturally be forced to hunt different prey; and from
the continued preservation of the individuals best fitted
for the two sites, two varieties might slowly be formed.
These varieties would cross and blend where they met; but
to this subject of intercrossing we shall soon have to return.
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.
Let us now take a more complex case. Certain plants excrete
a sweet juice, apparently for the sake of eliminating something
injurious from their sap: this is effected by glands at
the base of the stipules in some Leguminosae, and at the
back of the leaf of the common laurel. This juice, though
small in quantity, is greedily sought by insects. Let us
now suppose a little sweet juice or nectar to be excreted
by the inner bases of the petals of a flower. In this case
insects in seeking the nectar would get dusted with pollen,
and would certainly often transport the pollen from one
flower to the stigma of another flower. The flowers of two
distinct individuals of the same species would thus get
crossed; and the act of crossing, we have good reason to
believe (as will hereafter be more fully alluded to), would
produce very vigorous seedlings, which consequently would
have the best chance of flourishing and surviving. Some
of these seedlings would probably inherit the nectar-excreting
power. Those in individual flowers which had the largest
glands or nectaries, and which excreted most nectar, would
be oftenest visited by insects, and would be oftenest crossed;
and so in the long-run would gain the upper hand. Those
flowers, also, which had their stamens and pistils placed,
in relation to the size and habits of the particular insects
which visited them, so as to favour in any degree the transportal
of their pollen from flower to flower, would likewise be
favoured or selected. 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 object of
fertilisation, its destruction appears 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; and those individuals
which produced more and more pollen, and had larger and
larger anthers, would be selected.
When our plant, by this process of the continued preservation
or natural selection of more and more attractive flowers,
had been rendered highly attractive to insects, they would,
unintentionally on their part, regularly carry pollen from
flower to flower; and that they can most effectually do
this, I could easily show by many striking instances. I
will give only one not as a very striking case, but as likewise
illustrating one step in the separation of the sexes of
plants, presently to be alluded to. Some holly-trees bear
only male flowers, which have four stamens producing rather
a small quantity of pollen, and a rudimentary pistil; other
holly-trees bear only female flowers; these have a full-sized
pistil, and four stamens with shrivelled anthers, in which
not a grain of pollen can be detected. Having found a female
tree exactly sixty yards from a male tree, I put the stigmas
of twenty flowers, taken from different branches, under
the microscope, and on all, without exception, there were
pollen-grains, and on some a profusion of pollen. As the
wind had set for several days from the female to the male
tree, the pollen could not thus have been carried. The weather
had been cold and boisterous, and therefore not favourable
to bees, nevertheless every female flower which I examined
had been effectually fertilised by the bees, accidentally
dusted with pollen, having flown from tree to tree in search
of nectar. But to return to our imaginary case: as soon
as the plant had been rendered so highly attractive to insects
that pollen was regularly carried from flower to flower,
another process might commence. No naturalist doubts the
advantage of what has been called the 'physiological division
of labour;' hence we may believe that it would be advantageous
to a plant to produce stamens alone in one flower or on
one whole plant, and pistils alone in another flower or
on another plant. In plants under culture and placed under
new conditions of life, sometimes the male organs and sometimes
the female organs become more or less impotent; now if we
suppose this to occur in ever so slight a degree under nature,
then as pollen is already carried regularly from flower
to flower, and as a more complete separation of the sexes
of our plant would be advantageous on the principle of the
division of labour, individuals with this tendency more
and more increased, would be continually favoured or selected,
until at last a complete separation of the sexes would be
effected.
Let us now turn to the nectar-feeding insects in our imaginary
case: we may suppose the plant of which we have been slowly
increasing the nectar by continued selection, to be a common
plant; and that certain insects depended in main part on
its nectar for food. I could give many facts, showing how
anxious bees are to save time; for instance, their habit
of cutting holes and sucking the nectar at the bases of
certain flowers, which they can, with a very little more
trouble, enter by the mouth. Bearing such facts in mind,
I can see no reason to doubt that an accidental deviation
in the size and form of the body, or in the curvature and
length of the proboscis, &c., far too slight to be appreciated
by us, might profit a bee or other insect, so that an individual
so characterised would be able to obtain its food more quickly,
and so have a better chance of living and leaving descendants.
Its descendants would probably inherit a tendency to a similar
slight deviation of structure. The tubes of the corollas
of the common red and incarnate clovers (Trifolium pratense
and incarnatum) do not on a hasty glance appear to differ
in length; yet the hive-bee can easily suck the nectar out
of the incarnate clover, but not out of the common red clover,
which is visited by humble-bees alone; so that whole fields
of the red clover offer in vain an abundant supply of precious
nectar to the hive-bee. Thus it might be a great advantage
to the hive-bee to have a slightly longer or differently
constructed proboscis. On the other hand, I have found by
experiment that the fertility of clover greatly depends
on bees visiting and moving parts of the corolla, so as
to push the pollen on to the stigmatic surface. Hence, again,
if humble-bees were to become rare in any country, it might
be a great advantage to the red clover to have a shorter
or more deeply divided tube to its corolla, so that the
hive-bee could visit its flowers. Thus I can understand
how a flower and a bee might slowly become, either simultaneously
or one after the other, modified and adapted in the most
perfect manner to each other, by the continued preservation
of individuals presenting mutual and slightly favourable
deviations of structure.
I am well aware that this doctrine of natural selection,
exemplified in the above imaginary instances, is open to
the same objections which were at first urged against Sir
Charles Lyell's noble views on 'the modern changes of the
earth, as illustrative of geology;' but we now very seldom
hear the action, for instance, of the coast-waves, called
a trifling and insignificant cause, when applied to the
excavation of gigantic valleys or to the formation of the
longest lines of inland cliffs. Natural selection can act
only by the preservation and accumulation of infinitesimally
small inherited modifications, each profitable to the preserved
being; and as modern geology has almost banished such views
as the excavation of a great valley by a single diluvial
wave, so will natural selection, if it be a true principle,
banish the belief of the continued creation of new organic
beings, or of any great and sudden modification in their
structure.
On the Intercrossing of Individuals
I must here introduce a short digression. In the case
of animals and plants with separated sexes, it is of course
obvious that two individuals must always unite for each
birth; but in the case of hermaphrodites this is far from
obvious. Nevertheless I am strongly inclined to believe
that with all hermaphrodites two individuals, either occasionally
or habitually, concur for the reproduction of their kind.
This view, I may add, was first suggested by Andrew Knight.
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, 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 vigour and fertility
to the offspring; and on the other hand, that close
interbreeding diminishes vigour and fertility; that these
facts alone incline me to believe that it is a general law
of nature (utterly ignorant though we be of the meaning
of the law) that no organic being self-fertilises itself
for an eternity of generations; but that a cross with another
individual is occasionally perhaps at very long intervals
-- indispensable.
On the belief that this is a law of nature, we can, I
think, understand several large classes of facts, such as
the following, which on any other view are inexplicable.
Every hybridizer knows how unfavourable exposure to wet
is to the fertilisation of a flower, yet what a multitude
of flowers have their anthers and stigmas fully exposed
to the weather! but if an occasional cross be indispensable,
the fullest freedom for the entrance of pollen from another
individual will explain this state of exposure, more especially
as the plant's own anthers and pistil generally stand so
close together that self-fertilisation seems almost inevitable.
Many flowers, on the other hand, have their organs of fructification
closely enclosed, as in the great papilionaceous or pea-family;
but in several, perhaps in all, such flowers, there is a
very curious adaptation between the structure of the flower
and the manner in which bees suck the nectar; for, in doing
this, they either push the flower's own pollen on the stigma,
or bring pollen from another flower. So necessary are the
visits of bees to papilionaceous flowers, that I have found,
by experiments published elsewhere, that their fertility
is greatly diminished if these visits be prevented. Now,
it is scarcely possible that bees should fly from flower
to flower, and not carry pollen from one to the other, to
the great good, as I believe, of the plant. Bees will act
like a camel-hair pencil, and it is quite sufficient just
to touch the anthers of one flower and then the stigma of
another with the same brush to ensure fertilisation; but
it must not be supposed that bees would thus produce a multitude
of hybrids between distinct species; for if you bring on
the same brush a plant's own pollen and pollen from another
species, the former will have such a prepotent effect, that
it will invariably and completely destroy, as has been shown
by Gärtner, any influence from the foreign pollen.
When the stamens of a flower suddenly spring towards the
pistil, or slowly move one after the other towards it, the
contrivance seems adapted solely to ensure self-fertilisation;
and no doubt it is useful for this end: but, the agency
of insects is often required to cause the stamens to spring
forward, as Kölreuter has shown to be the case with
the barberry; and curiously in this very genus, which seems
to have a special contrivance for self-fertilisation, it
is well known that if very closely-allied forms or varieties
are planted near each other, it is hardly possible to raise
pure seedlings, so largely do they naturally cross. In many
other cases, far from there being any aids for self-fertilisation,
there are special contrivances, as I could show from the
writings of C. C. Sprengel and from my own observations,
which effectually prevent the stigma receiving pollen from
its own flower: for instance, in Lobelia fulgens, there
is a really beautiful and elaborate contrivance by which
every one of the infinitely numerous pollen-granules are
swept out of the conjoined anthers of each flower, before
the stigma of that individual flower is ready to receive
them; and as this flower is never visited, at least in my
garden, by insects, it never sets a seed, though by placing
pollen from one flower on the stigma of another, I raised
plenty of seedlings; and whilst another species of Lobelia
growing close by, which is visited by bees, seeds freely.
In very many other cases, though there be no special mechanical
contrivance to prevent the stigma of a flower receiving
its own pollen, yet, as C. C. Sprengel has shown, and as
I can confirm, either the anthers burst before the stigma
is ready for fertilisation, or the stigma is ready before
the pollen of that flower is ready, so that these plants
have in fact separated sexes, and must habitually be crossed.
How strange are these facts! How strange that the pollen
and stigmatic surface of the same flower, though placed
so close together, as if for the very purpose of self-fertilisation,
should in so many cases be mutually useless to each other!
How simply are these facts explained on the view of an occasional
cross with a distinct individual being advantageous or indispensable!
If several varieties of the cabbage, radish, onion, and
of some other plants, be allowed to seed near each other,
a large majority, as I have found, of the seedlings thus
raised will turn out mongrels: for instance, I raised 233
seedling cabbages from some plants of different varieties
growing near each other, and of these only 78 were true
to their kind, and some even of these were not perfectly
true. Yet the pistil of each cabbage-flower is surrounded
not only by its own six stamens, but by those of the many
other flowers on the same plant. How, then, comes it that
such a vast number of the seedlings are mongrelised? I suspect
that it must arise from the pollen of a distinct variety
having a prepotent effect over a flower's own pollen; and
that this is part of the general law of good being derived
from the intercrossing of distinct individuals of the same
species. When distinct species are crossed the case
is directly the reverse, for a plant's own pollen is always
prepotent over foreign pollen; but to this subject we shall
return in a future chapter.
In the case of a gigantic tree covered with innumerable
flowers, it may be objected that pollen could seldom be
carried from tree to tree, and at most only from flower
to flower on the same tree, and that flowers on the same
tree can be considered as distinct individuals only in a
limited sense. I believe this objection to be valid, but
that nature has largely provided against it by giving to
trees a strong tendency to bear flowers with separated sexes.
When the sexes are separated, although the male and female
flowers may be produced on the same tree, we can see that
pollen must be regularly carried from flower to flower;
and this will give a better chance of pollen being occasionally
carried from tree to tree. That trees belonging to all Orders
have their sexes more often separated than other plants,
I find to be the case in this country; and at my request
Dr Hooker tabulated the trees of New Zealand, and Dr Asa
Gray those of the United States, and the result was as I
anticipated. On the other hand, Dr Hooker has recently informed
me that he finds that the rule does not hold in Australia;
and I have made these few remarks on the sexes of trees
simply to call attention to the subject.
Turning for a very brief space to animals: on the land
there are some hermaphrodites, as land-mollusca and earth-worms;
but these all pair. As yet I have not found a single case
of a terrestrial animal which fertilises itself. We can
understand this remarkable fact, which offers so strong
a contrast with terrestrial plants, on the view of an occasional
cross being indispensable, by considering the medium in
which terrestrial animals live, and the nature of the fertilising
element; for we know of no means, analogous to the action
of insects and of the wind in the case of 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-fertilising hermaphrodites; but here
currents in the water offer an obvious means for an occasional
cross. And, 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 case of an
hermaphrodite animal with the organs of reproduction so
perfectly enclosed within the body, that access from without
and the occasional influence of a distinct individual can
be shown to be physically impossible. Cirripedes long appeared
to me to present a case of very great difficulty under this
point of view; but I have been enabled, by a fortunate chance,
elsewhere to prove that two individuals, though both are
self-fertilising hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly
that, in the case of both animals and plants, species of
the same family and even of the same genus, though agreeing
closely with each other in almost their whole organisation,
yet are not rarely, some of them hermaphrodites, and some
of them unisexual. But if, in fact, all hermaphrodites do
occasionally intercross with other individuals, the difference
between hermaphrodites and unisexual species, as far as
function is concerned, becomes very small.
From these several considerations and from the many special
facts which I have collected, but which I am not here able
to give, I am strongly inclined to suspect that, both in
the vegetable and animal kingdoms, an occasional intercross
with a distinct individual is a law of nature. I am well
aware that there are, on this view, many cases of difficulty,
some of which I am trying to investigate. Finally then,
we may conclude that in many organic beings, a cross between
two individuals is an obvious necessity for each birth;
in many others it occurs perhaps only at long intervals;
but in none, as I suspect, can self-fertilisation go on
for perpetuity.
Circumstances favourable to Natural Selection
This is an extremely intricate subject. A large amount
of inheritable and diversified variability is favourable,
but I believe mere individual differences suffice for the
work. A large number of individuals, by giving a better
chance for the appearance within any given period of profitable
variations, will compensate for a lesser amount of variability
in each individual, and is, I believe, an extremely important
element of success. Though nature grants vast periods of
time for the work of natural selection, she does not grant
an indefinite period; for as all organic beings are striving,
it may be said, to seize on each place in the economy of
nature, if any one species does not become modified and
improved in a corresponding degree with its competitors,
it will soon be exterminated.
In man's methodical selection, a breeder selects for some
definite object, and free intercrossing will wholly stop
his work. But when many men, without intending to alter
the breed, have a nearly common standard of perfection,
and all try to get and breed from the best animals, much
improvement and modification surely but slowly follow from
this unconscious process of selection, notwithstanding a
large amount of crossing with inferior animals. Thus it
will be in nature; for within a confined area, with some
place in its polity not so perfectly occupied as might be,
natural selection will always tend to preserve all the individuals
varying in the right direction, though in different degrees,
so as better to fill up the unoccupied place. But if the
area be large, its several districts will almost certainly
present different conditions of life; and then if natural
selection be modifying and improving a species in the several
districts, there will be intercrossing with the other individuals
of the same species on the confines of each. And in this
case the effects of intercrossing can hardly be counterbalanced
by natural selection always tending to modify all the individuals
in each district in exactly the same manner to the conditions
of each; for in a continuous area, the conditions will generally
graduate away insensibly from one district to another. The
intercrossing will most affect those animals which unite
for each birth, which wander much, and which do not breed
at a very quick rate. Hence in animals of this nature, for
instance in birds, varieties will generally be confined
to separated countries; and this I believe to be the case.
In hermaphrodite organisms which cross only occasionally,
and likewise in animals which unite for each birth, but
which wander little and which can increase at a very rapid
rate, a new and improved variety might be quickly formed
on any one spot, and might there maintain itself in a body,
so that whatever intercrossing took place would be chiefly
between the individuals of the same new variety. A local
variety when once thus formed might subsequently slowly
spread to other districts. On the above principle, nurserymen
always prefer getting seed from a large body of plants of
the same variety, as the chance of intercrossing with other
varieties is thus lessened.
Even in the case of slow-breeding animals, which unite
for each birth, we must not overrate the effects of intercrosses
in retarding natural selection; for I can bring a considerable
catalogue of facts, showing that within the same area, varieties
of the same animal can long remain distinct, from haunting
different stations, from breeding at slightly different
seasons, or from varieties of the same kind preferring to
pair together.
Intercrossing plays a very important part in nature in
keeping the individuals of the same species, or of the same
variety, true and uniform in character. It will obviously
thus act far more efficiently with those animals which unite
for each birth; but I have already attempted to show that
we have reason to believe that occasional intercrosses take
place with all animals and with all plants. Even if these
take place only at long intervals, I am convinced that the
young thus produced will gain so much in vigour and fertility
over the offspring from long-continued self-fertilisation,
that they will have a better chance of surviving and propagating
their kind; and thus, in the long run, the influence of
intercrosses, even at rare intervals, will be great. If
there exist organic beings which never intercross, uniformity
of character can be retained amongst them, as long as their
conditions of life remain the same, only through the principle
of inheritance, and through natural selection destroying
any which depart from the proper type; but if their conditions
of life change and they undergo modification, uniformity
of character can be given to their modified offspring, solely
by natural selection preserving the same favourable variations.
Isolation, also, is an important element in the process
of natural selection. In a confined or isolated area, if
not very large, the organic and inorganic conditions of
life will generally be in a great degree uniform; so that
natural selection will tend to modify all the individuals
of a varying species throughout the area in the same manner
in relation to the same conditions. Intercrosses, also,
with the individuals of the same species, which otherwise
would have inhabited the surrounding and differently circumstanced
districts, will be prevented. But isolation probably acts
more efficiently in checking the immigration of better adapted
organisms, after any physical change, such as of climate
or elevation of the land, &c.; and thus new places in
the natural economy of the country are left open for the
old inhabitants to struggle for, and become adapted to,
through modifications in their structure and constitution.
Lastly, isolation, by checking immigration and consequently
competition, will give time for any new variety to be slowly
improved; and this may sometimes be of importance in the
production of new species. If, however, an isolated area
be very small, either from being surrounded by barriers,
or from having very peculiar physical conditions, the total
number of the individuals supported on it will necessarily
be very small; and fewness of individuals will greatly retard
the production of new species through natural selection,
by decreasing the chance of the appearance of favourable
variations.
If we turn to nature to test the truth of these remarks,
and look at any small isolated area, such as an oceanic
island, although the total number of the species inhabiting
it, will be found to be small, as we shall see in our chapter
on geographical distribution; yet of these species a very
large proportion are endemic, that is, have been produced
there, and nowhere else. Hence an oceanic island at first
sight seems to have been highly favourable for the production
of new species. But we may thus greatly deceive ourselves,
for to ascertain whether a small isolated area, or a large
open area like a continent, has been most favourable for
the production of new organic forms, we ought to make the
comparison within equal times; and this we are incapable
of doing.
Although I do not doubt that isolation is of considerable
importance in the production of new species, on the whole
I am inclined to believe that largeness of area is of more
importance, more especially in the production of species,
which will prove capable of enduring for a long period,
and of spreading widely. Throughout a great and open area,
not only will there be a better chance of favourable variations
arising from the large number of individuals of the same
species there supported, but the conditions of life are
infinitely complex from the large number of already existing
species; and if some of these many species become modified
and improved, others will have to be improved in a corresponding
degree or they will be exterminated. Each new form, also,
as soon as it has been much improved, will be able to spread
over the open and continuous area, and will thus come into
competition with many others. Hence more new places will
be formed, and the competition to fill them will be more
severe, on a large than on a small and isolated area. Moreover,
great areas, though now continuous, owing to oscillations
of level, will often have recently existed in a broken condition,
so that the good effects of isolation will generally, to
a certain extent, have concurred. Finally, I conclude that,
although small isolated areas probably have been in some
respects highly favourable for the production of new species,
yet that the course of modification will generally have
been more rapid on large areas; and what is more important,
that the new forms produced on large areas, which already
have been victorious over many competitors, will be those
that will spread most widely, will give rise to most new
varieties and species, and will thus play an important part
in the changing history of the organic world.
We can, perhaps, on these views, understand some facts
which will be again alluded to in our chapter on geographical
distribution; for instance, that the productions of the
smaller continent of Australia have formerly yielded, and
apparently are now yielding, before those of the larger
Europaeo-Asiatic area. Thus, also, it is that continental
productions have everywhere become so largely naturalised
on islands. On a small island, the race for life will have
been less severe, and there will have been less modification
and less extermination. Hence, perhaps, it comes that the
flora of Madeira, according to Oswald Heer, resembles the
extinct tertiary flora of Europe. All fresh-water basins,
taken together, make a small area compared with that of
the sea or of the land; and, consequently, the competition
between fresh-water productions will have been less severe
than elsewhere; new forms will have been more slowly formed,
and old forms more slowly exterminated. And it is in fresh
water that we find seven genera of Ganoid fishes, remnants
of a once preponderant order: and in fresh water we find
some of the most anomalous forms now known in the world,
as the Ornithorhynchus and Lepidosiren, which, like fossils,
connect to a certain extent orders now widely separated
in the natural scale. These anomalous forms may almost be
called living fossils; they have endured to the present
day, from having inhabited a confined area, and from having
thus been exposed to less severe competition.
To sum up the circumstances favourable and unfavourable
to natural selection, as far as the extreme intricacy of
the subject permits. I conclude, looking to the future,
that for terrestrial productions a large continental area,
which will probably undergo many oscillations of level,
and which consequently will exist for long periods in a
broken condition, will be the most favourable for the production
of many new forms of life, likely to endure long and to
spread widely. For the area will first have existed as a
continent, and the inhabitants, at this period numerous
in individuals and kinds, will have been subjected to very
severe competition. When converted by subsidence into large
separate islands, there will still exist many individuals
of the same species on each island: intercrossing on the
confines of the range of each species will thus be checked:
after physical changes of any kind, immigration will be
prevented, so that new places in the polity of each island
will have to be filled up by modifications of the old inhabitants;
and time will be allowed for the varieties in each to become
well modified and perfected. When, by renewed elevation,
the islands shall be re-converted into a continental area,
there will again be severe competition: the most favoured
or improved varieties will be enabled to spread: there will
be much extinction of the less improved forms, and the relative
proportional numbers of the various inhabitants of the renewed
continent will again be changed; and again there will be
a fair field for natural selection to improve still further
the inhabitants, and thus produce new species.
That natural selection will always act with extreme slowness,
I fully admit. Its action depends on there being places
in the polity of nature, which can be better occupied by
some of the inhabitants of the country undergoing modification
of some kind. The existence of such places will often depend
on physical changes, which are generally very slow, and
on the immigration of better adapted forms having been checked.
But the action of natural selection will probably still
oftener depend on some of the inhabitants becoming slowly
modified; the mutual relations of many of the other inhabitants
being thus disturbed. Nothing can be effected, unless favourable
variations occur, and variation itself is apparently always
a very slow process. The process will often be greatly retarded
by free intercrossing. Many will exclaim that these several
causes are amply sufficient wholly to stop the action of
natural selection. I do not believe so. On the other hand,
I do believe that natural selection will always act very
slowly, often only at long intervals of time, and generally
on only a very few of the inhabitants of the same region
at the same time. I further believe, that this very slow,
intermittent action of natural selection accords perfectly
well with what geology tells us of the rate and manner at
which the inhabitants of this world have changed.
Slow though the process of selection may be, if feeble
man can do much by his powers of artificial selection, I
can see no limit to the amount of change, to the beauty
and infinite complexity of the coadaptations between all
organic beings, one with another and with their physical
conditions of life, which may be effected in the long course
of time by nature's power of selection.
Extinction
This subject will be more fully discussed in our chapter
on Geology; but it must be here alluded to from being intimately
connected with natural selection. Natural selection acts
solely through the preservation of variations in some way
advantageous, which consequently endure. But as from the
high geometrical powers of increase of all organic beings,
each area is already fully stocked with inhabitants, it
follows that as each selected and favoured form increases
in number, so will the less favoured forms decrease and
become rare. Rarity, as geology tells us, is the precursor
to extinction. We can, also, see that any form represented
by few individuals will, during fluctuations in the seasons
or in the number of its enemies, run a good chance of utter
extinction. But we may go further than this; for as new
forms are continually and slowly being produced, unless
we believe that the number of specific forms goes on perpetually
and almost indefinitely increasing, numbers inevitably must
become extinct. That the number of specific forms has not
indefinitely increased, geology shows us plainly; and indeed
we can see reason why they should not have thus increased,
for the number of places in the polity of nature is not
indefinitely great, not that we have any means of knowing
that any one region has as yet got its maximum of species.
probably no region is as yet fully stocked, for at the Cape
of Good Hope, where more species of plants are crowded together
than in any other quarter of the world, some foreign plants
have become naturalised, without causing, as far as we know,
the extinction of any natives.
Furthermore, the species which are most numerous in individuals
will have the best chance of producing within any given
period favourable variations. We have evidence of this,
in the facts given in the second chapter, showing that it
is the common species which afford the greatest number of
recorded varieties, or incipient species. Hence, rare species
will be less quickly modified or improved within any given
period, and they will consequently be beaten in the race
for life by the modified 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 amongst 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
The principle, which I have designated by this term, is
of high importance on my theory, 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 how to rank them yet certainly differ from
each other far less 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 an amount of difference as that
between varieties of the same species and species of the
same genus.
As has always been my practice, let us seek light on this
head from our domestic productions. We shall here find something
analogous. A fancier is 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 tumbler-pigeons) 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 one man preferred
swifter horses; another stronger and more bulky horses.
The early differences would be very slight; in the course
of time, from the continued selection of swifter horses
by some breeders, and of stronger ones by others, the differences
would become greater, and would be noted as forming two
sub-breeds; finally, after the lapse of centuries, the sub-breeds
would become converted into two well-established and distinct
breeds. As the differences slowly become greater, the inferior
animals with intermediate characters, being neither very
swift nor very strong, will have been neglected, and will
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,
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 see 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 powers of
increase be allowed to act, it can succeed in increasing
(the country not undergoing any change in its 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 animal became, the more places they would 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 do nothing. So it will
be with plants. It has been experimentally proved, that
if a plot of ground be sown with several distinct genera
of grasses, a greater number of plants and a greater weight
of dry herbage can thus be raised. The same has been found
to hold good when first one variety and then several mixed
varieties of wheat have been sown on equal spaces of ground.
Hence, if any one species of grass were to go on varying,
and those varieties were continually selected which differed
from each other in at all the same manner as distinct species
and genera of grasses differ from each other, a greater
number of individual plants of this species of grass, including
its modified descendants, would succeed in living on the
same piece of ground. And we well know that each species
and each variety of grass is annually sowing almost countless
seeds; and thus, as it may be said, is striving its utmost
to increase its numbers. Consequently, I cannot doubt that
in the course of many thousands of generations, the most
distinct varieties of any one species of grass would always
have the best chance of succeeding and of increasing in
numbers, and thus of supplanting the less distinct varieties;
and varieties, when rendered very distinct from each other,
take the rank of species.
The truth of the principle, that the greatest amount of
life can be supported by great diversification of structure,
is seen under many natural circumstances. In an extremely
small area, especially if freely open to immigration, and
where the contest between individual and individual must
be severe, we always find great diversity in its inhabitants.
For instance, I found that a piece of turf, three feet by
four in size, which had been exposed for many years to exactly
the same conditions, supported twenty species of plants,
and these belonged to eighteen genera and to eight orders,
which shows how much these plants differed from each other.
So it is with the plants and insects on small and uniform
islets; and so in small ponds of fresh water. Farmers find
that they can raise most food by a rotation of plants belonging
to the most different orders: nature follows what may be
called a simultaneous rotation. Most of the animals and
plants which live close round any small piece of ground,
could live on it (supposing it not to be in any way peculiar
in its nature), and may be said to be striving to the utmost
to live there; but, it is seen, that where they come into
the closest competition with each other, the advantages
of diversification of structure, with the accompanying differences
of habit and constitution, determine that the inhabitants,
which thus jostle each other most closely, shall, as a general
rule, belong to what we call different genera and orders.
The same principle is seen in the naturalisation of plants
through man's agency in foreign lands. It might have been
expected that the plants which have succeeded in becoming
naturalised in any land would generally have been closely
allied to the indigenes; for these are commonly looked at
as specially created and adapted for their own country.
It might, also, perhaps have been expected that naturalised
plants would have belonged to a few groups more especially
adapted to certain stations in their new homes. But the
case is very different; and Alph. De Candolle has well remarked
in his great and admirable work, that floras gain by naturalisation,
proportionally with the number of the native genera and
species, far more in new genera than in new species. To
give a single instance: in the last edition of Dr Asa Gray's
'Manual of the Flora of the Northern United States,' 260
naturalised plants are enumerated, and these belong to 162
genera. We thus see that these naturalised plants are of
a highly diversified nature. They differ, moreover, to a
large extent from the indigenes, for out of the 162 genera,
no less than 100 genera are not there indigenous, and thus
a large proportional addition is made to the genera of these
States.
By considering the nature of the plants or animals which
have struggled successfully with the indigenes of any country,
and have there become naturalised, we can gain some crude
idea in what manner some of the natives would have had to
be modified, in order to have gained an advantage over the
other natives; and we may, I think, at least safely infer
that diversification of structure, amounting to new generic
differences, would have been profitable to them.
The advantage of diversification in the inhabitants of
the same region is, in fact, the same as that of the physiological
division of labour in the organs of the same individual
body a subject so well elucidated by Milne Edwards. No physiologist
doubts that a stomach by being 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 organisation 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-pronounced orders. In the Australian mammals,
we see the process of diversification in an early and incomplete
stage of development.
After the foregoing discussion, which ought to have been
much amplified, we may, I think, assume that the modified
descendants of any one species will succeed by so much the
better as they become more diversified in structure, and
are thus enabled to encroach on places occupied by other
beings. Now let us see how this principle of great benefit
being derived from divergence of character, combined with
the principles of natural selection and of extinction, will
tend to act.
The accompanying diagram will aid us in understanding
this rather perplexing subject. Let A to L represent the
species of a genus large in its own country; these species
are supposed to resemble each other in unequal degrees,
as is so generally the case in nature, and as is represented
in the diagram by the letters standing at unequal distances.
I have said a large genus, because we have seen in the second
chapter, that on an average more of the species of large
genera vary than of small genera; and the varying species
of the large genera present a greater number of varieties.
We have, also, seen that the species, which are the commonest
and the most widely-diffused, vary more than rare species
with restricted ranges. Let (A) be a common, widely-diffused,
and varying species, belonging to a genus large in its own
country. The little fan of diverging dotted lines of unequal
lengths proceeding from (A), may represent its varying offspring.
The variations are supposed to be extremely slight, but
of the most diversified nature; they are not supposed all
to appear simultaneously, but often after long intervals
of time; nor are they all supposed to endure for equal periods.
Only those variations which are in some way profitable will
be preserved or naturally selected. And here the importance
of the principle of benefit being derived from divergence
of character comes in; for this will generally lead to the
most different or divergent variations (represented by the
outer dotted lines) being preserved and accumulated by natural
selection. When a dotted line reaches one of the horizontal
lines, and is there marked by a small numbered letter, a
sufficient amount of variation is supposed to have been
accumulated to have formed a fairly well-marked variety,
such as would be thought worthy of record in a systematic
work.
The intervals between the horizontal lines in the diagram,
may represent each a thousand generations; but it would
have been better if each had represented ten thousand generations.
After a thousand generations, species (A) is supposed to
have produced two fairly well-marked varieties, namely a1
and m1. These two varieties will generally continue
to be exposed to the same conditions which made their parents
variable, and the tendency to variability is in itself hereditary,
consequently they will tend to vary, and generally to vary
in nearly the same manner as their parents varied. Moreover,
these two varieties, being only slightly modified forms,
will tend to inherit those advantages which made their common
parent (A) more numerous than most of the other inhabitants
of the same country; they will likewise partake of those
more general advantages which made the genus to which the
parent-species belonged, a large genus in its own country.
And these circumstances we know to be favourable to the
production of new varieties.
If, then, these two varieties be variable, the most divergent
of their variations will generally be preserved during the
next thousand generations. And after this interval, variety
a1 is supposed in the diagram to have produced variety
a2, which will, owing to the principle of divergence,
differ more from (A) than did variety a1. Variety
m1 is supposed to have produced two varieties, namely
m 2 and s2, differing from each other, and
more considerably from their common parent (A). We may continue
the process by similar steps for any length of time; some
of the varieties, after each thousand generations, producing
only a single variety, but in a more and more modified condition,
some producing two or three varieties, and some failing
to produce any. Thus the varieties or modified descendants,
proceeding from the common parent (A), will generally go
on increasing in number and diverging in character. In the
diagram the process is represented up to the ten-thousandth
generation, and under a condensed and simplified form up
to the fourteen-thousandth generation.
But I must here remark that I do not suppose that the
process ever goes on so regularly as is represented in the
diagram, though in itself made somewhat irregular. I am
far from thinking that the most divergent varieties will
invariably prevail and multiply: a medium form may often
long endure, and may or may not produce more than one modified
descendant; for natural selection will always act according
to the nature of the places which are either unoccupied
or not perfectly occupied by other beings; and this will
depend on infinitely complex relations. But as a general
rule, the more diversified in structure the descendants
from any one species can be rendered, the more places they
will be enabled to seize on, and the more their modified
progeny will be increased. In our diagram the line of succession
is broken at regular intervals by small numbered letters
marking the successive forms which have become sufficiently
distinct to be recorded as varieties. But these breaks are
imaginary, and might have been inserted anywhere, after
intervals long enough to have allowed the accumulation of
a considerable amount of divergent variation.
As all the modified descendants from a common and widely-diffused
species, belonging to a large genus, will tend to partake
of the same advantages which made their parent successful
in life, they will generally go on multiplying in number
as well as diverging in character: this is represented in
the diagram by the several divergent branches proceeding
from (A). The modified offspring from the later and more
highly improved branches in the lines of descent, will,
it is probable, often take the place of, and so destroy,
the earlier and less improved branches: this is represented
in the diagram by some of the lower branches not reaching
to the upper horizontal lines. In some cases I do not doubt
that the process of modification will be confined to a single
line of descent, and the number of the descendants will
not be increased; although the amount of divergent modification
may have been increased in the successive generations. This
case would be represented in the diagram, if all the lines
proceeding from (A) were removed, excepting that from a1
to a10 In the same way, for instance, the English
race-horse and English pointer have apparently both gone
on slowly diverging in character from their original stocks,
without either having given off any fresh branches or races.
After ten thousand generations, species (A) is supposed
to have produced three forms, a10, f10, and
m10, which, from having diverged in character during
the successive generations, will have come to differ largely,
but perhaps unequally, from each other and from their common
parent. If we suppose the amount of change between each
horizontal line in our diagram to be excessively small,
these three forms may still be only well-marked varieties;
or they may have arrived at the doubtful category of sub-species;
but we have only to suppose the steps in the process of
modification to be more numerous or greater in amount, to
convert these three forms into well-defined species: thus
the diagram illustrates the steps by which the small differences
distinguishing varieties are increased into the larger differences
distinguishing species. By continuing the same process for
a greater number of generations (as shown in the diagram
in a condensed and simplified manner), we get eight species,
marked by the letters between a14 and m14,
all descended from (A). Thus, as I believe, species are
multiplied and genera are formed.
In a large genus it is probable that more than one species
would vary. In the diagram I have assumed that a second
species (I) has produced, by analogous steps, after ten
thousand generations, either two well-marked varieties (w10
and z10) or two species, according to the amount
of change supposed to be represented between the horizontal
lines. After fourteen thousand generations, six new species,
marked by the letters n14 to z14, are supposed
to have been produced. In each genus, the species, which
are already extremely different in character, will generally
tend to produce the greatest number of modified descendants;
for these will have the best chance of filling new and widely
different places in the polity of nature: hence in the diagram
I have chosen the extreme species (A), and the nearly extreme
species (I), as those which have largely varied, and have
given rise to new varieties and species. The other nine
species (marked by capital letters) of our original genus,
may for a long period continue transmitting unaltered descendants;
and this is shown in the diagram by the dotted lines not
prolonged far upwards from want of space.
But during the process of modification, represented in
the diagram, another of our principles, namely that of extinction,
will have played an important part. As in each fully stocked
country natural selection necessarily acts by the selected
form having some advantage in the struggle for life over
other forms, there will be a constant tendency in the improved
descendants of any one species to supplant and exterminate
in each stage of descent their predecessors and their original
parent. For it should be remembered that the competition
will generally be most severe between those forms which
are most nearly related to each other in habits, constitution,
and structure. Hence all the intermediate forms between
the earlier and later states, that is between the less and
more improved state of a species, as well as the original
parent-species itself, will generally tend to become extinct.
So it probably will be with many whole collateral lines
of descent, which will be conquered by later and improved
lines of descent. If, however, the modified offspring of
a species get into some distinct country, or become quickly
adapted to some quite new station, in which child and parent
do not come into competition, both may continue to exist.
If then our diagram be assumed to represent a considerable
amount of modification, species (A) and all the earlier
varieties will have become extinct, having been replaced
by eight new species (a14 to m14); and (I)
will have been replaced by six (n14 to z14)
new species.
But we may go further than this. The original species
of our genus were supposed to resemble each other in unequal
degrees, as is so generally the case in nature; species
(A) being more nearly related to B, C, and D, than to the
other species; and species (I) more to G, H, K, L, than
to the others. These two species (A) and (I), were also
supposed to be very common and widely diffused species,
so that they must originally have had some advantage over
most of the other species of the genus. Their modified descendants,
fourteen in number at the fourteen-thousandth generation,
will probably have inherited some of the same advantages:
they have also been modified and improved in a diversified
manner at each stage of descent, so as to have become adapted
to many related places in the natural economy of their country.
It seems, therefore, to me extremely probable that they
will have taken the places of, and thus exterminated, not
only their parents (A) and (I), but likewise some of the
original species which were most nearly related to their
parents. Hence very few of the original species will have
transmitted offspring to the fourteen-thousandth generation.
We may suppose that only one (F), of the two species which
were least closely related to the other nine original species,
has transmitted descendants to this late stage of descent.
The new species in our diagram descended from the original
eleven species, will now be fifteen in number. Owing to
the divergent tendency of natural selection, the extreme
amount of difference in character between species a14
and z14 will be much greater than that between the
most different of the original eleven species. The new species,
moreover, will be allied to each other in a widely different
manner. Of the eight descendants from (A) the three marked
a14, q14, p14, will be nearly related
from having recently branched off from a14; b14
and f14, from having diverged at an earlier period
from a5, will be in some degree distinct from the
three first-named species; and lastly, o14, e14,
and m14, will be nearly related one to the other,
but from having diverged at the first commencement of the
process of modification, will be widely different from the
other five species, and may constitute a sub-genus or even
a distinct genus. The six descendants from (I) will
form two sub-genera or even genera. But as the original
species (I) differed largely from (A), standing nearly at
the extreme points of the original genus, the six descendants
from (I) will, owing to inheritance, differ considerably
from the eight descendants from (A); the two groups, moreover,
are supposed to have gone on diverging in different directions.
The intermediate species, also (and this is a very important
consideration), which connected the original species (A)
and (I), have all become, excepting (F), extinct, and have
left no descendants. Hence the six new species descended
from (I), and the eight descended from (A), will have to
be ranked as very distinct genera, or even as distinct sub-families.
Thus it is, as I believe, that two or more genera are
produced by descent, with modification, from two or more
species of the same genus. And the two or more parent-species
are supposed to have descended from some one species of
an earlier genus. In our diagram, this is indicated by the
broken lines, beneath the capital letters, converging in
sub-branches downwards towards a single point; this point
representing a single species, the supposed single parent
of our several new sub-genera and genera.
It is worth while to reflect for a moment on the character
of the new species F14, which is supposed not to have diverged
much in character, but to have retained the form of (F),
either unaltered or altered only in a slight degree. In
this case, its affinities to the other fourteen new species
will be of a curious and circuitous nature. Having descended
from a form which stood between the two parent-species (A)
and (I), now supposed to be extinct and unknown, it will
be in some degree intermediate in character between the
two groups descended from these species. But as these two
groups have gone on diverging in character from the type
of their parents, the new species (F14) will not be directly
intermediate between them, but rather between types of the
two groups; and every naturalist will be able to bring some
such case before his mind.
In the diagram, each horizontal line has hitherto been
supposed to represent a thousand generations, but each may
represent a million or hundred million generations, and
likewise a section of the successive strata of the earth's
crust including extinct remains. We shall, when we come
to our chapter on Geology, have to refer again to this subject,
and I think we shall then see that the diagram throws light
on the affinities of extinct beings, which, though generally
belonging to the same orders, or families, or genera, with
those now living, yet are often, in some degree, intermediate
in character between existing groups; and we can understand
this fact, for the extinct species lived at very ancient
epochs when the branching lines of descent had diverged
less.
I see no reason to limit the process of modification,
as now explained, to the formation of genera alone. If,
in our diagram, we suppose the amount of change represented
by each successive group of diverging dotted lines to be
very great, the forms marked a214 to p14, those marked
b14 and f14, and those marked o14 to
m14, will form three very distinct genera. We shall
also have two very distinct genera descended from (I) and
as these latter two genera, both from continued divergence
of character and from inheritance from a different parent,
will differ widely from the three genera descended from
(A), the two little groups of genera will form two distinct
families, or even orders, according to the amount of divergent
modification supposed to be represented in the diagram.
And the two new families, or orders, will have descended
from two species of the original genus; and these two species
are supposed to have descended from one species of a still
more ancient and unknown genus.
We have seen that in each country it is the species of
the larger genera which oftenest present varieties or incipient
species. This, indeed, might have been expected; for as
natural selection acts through one form having some advantage
over other forms in the struggle for existence, it will
chiefly act on those which already have some advantage;
and the largeness of any group shows that its species have
inherited from a common ancestor some advantage in common.
Hence, the struggle for the production of new and modified
descendants, will mainly lie between the larger groups,
which are all trying to increase in number. One large group
will slowly conquer another large group, reduce its numbers,
and thus lessen its chance of further variation and improvement.
Within the same large group, the later and more highly perfected
sub-groups, from branching out and seizing on many new places
in the polity of Nature, will constantly tend to supplant
and destroy the earlier and less improved sub-groups. Small
and broken groups and sub-groups will finally tend to disappear.
Looking to the future, we can predict that the groups of
organic beings which are now large and triumphant, and which
are least broken up, that is, which as yet have suffered
least extinction, will for a long period continue to increase.
But which groups will ultimately prevail, no man can predict;
for we well know that many groups, formerly most extensively
developed, have now become extinct. Looking still more remotely
to the future, we may predict that, owing to the continued
and steady increase of the larger groups, a multitude of
smaller groups will become utterly extinct, and leave no
modified descendants; and consequently that of the species
living at any one period, extremely few will transmit descendants
to a remote futurity. I shall have to return to this subject
in the chapter on Classification, but I may add that on
this view of extremely few of the more ancient species having
transmitted descendants, and on the view of all the descendants
of the same species making a class, we can understand how
it is that there exist but very few classes in each main
division of the animal and vegetable kingdoms. Although
extremely few of the most ancient species may now have living
and modified descendants, yet at the most remote geological
period, the earth may have been as well peopled with many
species of many genera, families, orders, and classes, as
at the present day.
Summary of Chapter
If during the long course of ages and under varying conditions
of life, organic beings vary at all in the several parts
of their organisation, and I think this cannot be disputed;
if there be, owing to the high geometrical powers of increase
of each species, at some age, season, or year, a severe
struggle for life, and this certainly cannot be disputed;
then, considering the infinite complexity of the relations
of all organic beings to each other and to their conditions
of existence, causing an infinite diversity in structure,
constitution, and habits, to be advantageous to them, I
think it would be a most extraordinary fact if no variation
ever had occurred useful to each being's own welfare, in
the same way as so many variations have occurred useful
to man. But if variations useful to any organic being do
occur, assuredly individuals thus characterised |
