- 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
will have the best chance of being preserved in the struggle for life; and
from the strong principle of inheritance they will tend to produce offspring
similarly characterised. This principle of preservation, I have called, for
the sake of brevity, Natural Selection. Natural selection, on the principle
of qualities being inherited at corresponding ages, can modify the egg, seed,
or young, as easily as the adult. Amongst many animals, sexual selection will
give 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 with other males.
Whether natural selection has really thus acted in nature, in modifying and
adapting the various forms of life to their several conditions and stations,
must be judged of by the general tenour and balance of evidence given in the
following chapters. But we already see 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 more living beings can
be supported on the same area the more they diverge in structure, habits, and
constitution, of which we see proof by looking at the inhabitants of any small
spot or at naturalised productions. 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 these descendants become, the
better will be their chance of succeeding in the battle of life. Thus the small
differences distinguishing varieties of the same species, will steadily tend
to increase till they come to 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, which vary most; and these will 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, I believe, the nature
of the affinities of all organic beings 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 group subordinate to group, in the manner which we everywhere behold namely,
varieties of the same species most closely related together, species of the
same genus less closely and unequally related together, 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 cannot be ranked in
a single file, but seem rather to be clustered round points, and these round
other points, and so on in almost endless cycles. On the view that each species
has been independently created, I can see no explanation of this great fact
in the classification of all organic beings; but, to the best of my judgment,
it is explained through inheritance and the complex action of natural selection,
entailing extinction and divergence of character, as we have seen illustrated
in the diagram.
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
each former year 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 tried to overmaster 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 small,
budding twigs; and this connexion 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 all the other branches; so with the species which lived during
long-past geological periods, very few now have living and modified descendants.
From the first growth of the tree, many a limb and branch has decayed and dropped
off; and these lost 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 from having been found 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 favoured 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. |