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Chapter IV
NATURAL SELECTION; OR THE SURVIVAL OF THE FITTEST.
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 the results of 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 -- Advance in organisation -- Low forms preserved -- Convergence of
character -- Indefinite multiplication of species -- Summary.
How will the struggle for existence, briefly discussed 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 under nature? I think we
shall see that it can act most efficiently. Let the endless number of
slight variations and individual differences occurring in our domestic
productions, and, in a lesser degree, in those under nature, be borne in
mind; as well as the strength of the hereditary tendency. Under
domestication, it may truly be said that the whole organisation becomes in
some degree plastic. But the variability, which we almost universally meet
with in our domestic productions is not directly produced, as Hooker and
Asa Gray have well remarked, by man; he can neither originate varieties nor
prevent their occurrence; he can only preserve and accumulate such as do
occur. Unintentionally he exposes organic beings to new and changing
conditions of life, and variability ensues; but similar changes of
conditions might and do occur under nature. Let it also 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; and
consequently what infinitely varied diversities of structure might be of
use to each being under changing 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 occur in the course of many
successive 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 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 individual differences and
variations, and the destruction of those which are injurious, I have called
Natural Selection, or the Survival of the Fittest. Variations neither
useful nor injurious would not be affected by natural selection, and would
be left either a fluctuating element, as perhaps we see in certain
polymorphic species, or would ultimately become fixed, owing to the nature
of the organism and the nature of the conditions.
Several writers have misapprehended or objected to the term Natural
Selection. Some have even imagined that natural selection induces
variability, whereas it implies only the preservation of such variations as
arise and are beneficial to the being under its conditions of life. No one
objects to agriculturists speaking of the potent effects of man's
selection; and in this case the individual differences given by nature,
which man for some object selects, must of necessity first occur. Others
have objected that the term selection implies conscious choice in the
animals which become modified; and it has even been urged that, as plants
have no volition, natural selection is not applicable to them! In the
literal sense of the word, no doubt, natural selection is a false term; but
who ever objected to chemists speaking of the elective affinities of the
various elements?--and yet an acid cannot strictly be said to elect the
base with which it in preference combines. It has been said that I speak
of natural selection as an active power or Deity; but who objects to an
author speaking of the attraction of gravity as ruling the movements of the
planets? Every one knows what is meant and is implied by such metaphorical
expressions; and they are almost necessary for brevity. So again it is
difficult to avoid personifying the word Nature; but I mean by nature, only
the aggregate action and product of many natural laws, and by laws the
sequence of events as ascertained by us. With a little familiarity such
superficial objections will be forgotten.
We shall best understand the probable course of natural selection by taking
the case of a country undergoing some slight physical change, for instance,
of climate. The proportional numbers of its inhabitants will almost
immediately undergo a change, and some species will probably 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 the inhabitants,
independently of the change of climate itself, would seriously affect the
others. If the country were open on its borders, new forms would certainly
immigrate, and this would likewise 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 cases, slight modifications, which in any
way favoured the individuals of any species, by better adapting them to
their altered conditions, would tend to be preserved; and natural selection
would have free scope for the work of improvement.
We have good reason to believe, as shown in the first chapter, that changes
in the conditions of life give a tendency to increased variability; and in
the foregoing cases the conditions the changed, and this would manifestly
be favourable to natural selection, by affording a better chance of the
occurrence of profitable variations. Unless such occur, natural selection
can do nothing. Under the term of "variations," it must never be forgotten
that mere individual differences are included. As man can produce a great
result with his domestic animals and plants by adding up in any given
direction individual differences, so could natural selection, but far more
easily from having incomparably longer time for action. Nor do I believe
that any great physical change, as of climate, or any unusual degree of
isolation, to check immigration, is necessary in order that new and
unoccupied places should be left for natural selection to fill up by
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 species would often
give it an advantage over others; and still further modifications of the
same kind would often still further increase the advantage, as long as the
species continued under the same conditions of life and profited by similar
means of subsistence and defence. 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 be still
better adapted or improved; for in all countries, the natives have been so
far conquered by naturalised productions that they have allowed some
foreigners to take firm possession of the land. And as foreigners have
thus in every country beaten some of the natives, we may safely conclude
that the natives might have been modified with advantage, so as to have
better resisted the intruders.
As man can produce, and certainly has produced, a great result by his
methodical and unconscious means of selection, what may not natural
selection effect? Man can act only on external and visible characters:
Nature, if I may be allowed to personify the natural preservation or
survival of the fittest, cares nothing for appearances, except in so far as
they are 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, as is
implied by the fact of their selection. 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; 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 the eye or to be
plainly useful to him. Under nature, the slightest differences 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 be his
results, 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 metaphorically be said that natural selection is daily and hourly
scrutinising, throughout the world, the slightest variations; rejecting
those that are bad, preserving and adding up all that are 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 lapse of ages, and then so
imperfect is our view into long-past geological ages that we see only that
the forms of life are now different from what they formerly were.
In order that any great amount of modification should be effected in a
species, a variety, when once formed must again, perhaps after a long
interval of time, vary or present individual differences of the same
favourable nature as before; and these must again be preserved, and so
onward, step by step. Seeing that individual differences of the same kind
perpetually recur, this can hardly be considered as an unwarrantable
assumption. But whether it is true, we can judge only by seeing how far
the hypothesis accords with and explains the general phenomena of nature.
On the other hand, the ordinary belief that the amount of possible
variation is a strictly limited quantity, is likewise a simple assumption.
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, 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 natural selection might be
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 a lamb with the faintest trace of
black. We have seen how the colour of hogs, which feed on the "paint-root"
in Virginia, determines whether they shall live or die. 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 a
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 quite unimportant, we must
not forget that climate, food, etc., have no doubt produced some direct
effect. It is also necessary to bear in mind that, owing to the law of
correlation, when one part varies and the variations are accumulated
through natural selection, other modifications, often of the most
unexpected nature, will ensue.
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 shape, size and flavour of 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 variations profitable 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; and these modifications may affect, through
correlation, the structure of the adult. So, conversely, modifications in
the adult may affect the structure of the larva; but in all cases natural
selection will ensure that they shall not be injurious: for if they were
so, the species would become extinct.
Natural selection will modify the structure of the young in relation to the
parent and of the parent in relation to the young. In social animals it
will adapt the structure of each individual for the benefit of the whole
community; if the community profits by the selected change. What natural
selection 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 life, if of high importance to it, might be modified to
any extent by natural selection; for instance, the great jaws possessed by
certain insects, used exclusively for opening the cocoon--or the hard tip
to the beak of unhatched birds, used for breaking the eggs. It has been
asserted, that of the best short-beaked tumbler-pigeons a greater number
perish in the egg than are able to get out of it; so that fanciers assist
in the act of hatching. Now, if nature had to make the beak of a
full-grown pigeon very short for the bird's own advantage, the process of
modification would be very slow, and there would be simultaneously the most
rigorous selection of all the young birds within the egg, which had the
most powerful and hardest beaks, for all with weak beaks would inevitably
perish: or, more delicate and more easily broken shells might be selected,
the thickness of the shell being known to vary like every other structure.
It may be well here to remark that with all beings there must be much
fortuitous destruction, which can have little or no influence on the course
of natural selection. For instance, a vast number of eggs or seeds are
annually devoured, and these could be modified through natural selection
only if they varied in some manner which protected them from their enemies.
Yet many of these eggs or seeds would perhaps, if not destroyed, have
yielded individuals better adapted to their conditions of life than any of
those which happened to survive. So again a vast number of mature animals
and plants, whether or not they be the best adapted to their conditions,
must be annually destroyed by accidental causes, which would not be in the
least degree mitigated by certain changes of structure or constitution
which would in other ways be beneficial to the species. But let the
destruction of the adults be ever so heavy, if the number which can exist
in any district be not wholly kept down by such causes--or again let the
destruction of eggs or seeds be so great that only a hundredth or a
thousandth part are developed--yet of those which do survive, the best
adapted individuals, supposing that there is any variability in a
favourable direction, will tend to propagate their kind in larger numbers
than the less well adapted. If the numbers be wholly kept down by the
causes just indicated, as will often have been the case, natural selection
will be powerless in certain beneficial directions; but this is no valid
objection to its efficiency at other times and in other ways; for we are
far from having any reason to suppose that many species ever undergo
modification and improvement at the same time in the same area.
SEXUAL SELECTION.
Inasmuch as peculiarities often appear under domestication in one sex and
become hereditarily attached to that sex, so no doubt it will be under
nature. Thus it is rendered possible for the two sexes to be modified
through natural selection in relation to different habits of life, as is
sometimes the case; or for one sex to be modified in relation to the other
sex, as commonly occurs. This leads me to say a few words on what I have
called sexual selection. This form of selection depends, not on a struggle
for existence in relation to other organic beings or to external
conditions, but on a struggle between the individuals of one sex, generally
the males, for the possession of the other sex. 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 depends not so much 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 numerous
offspring. Sexual selection, by always allowing the victor to breed, might
surely give indomitable courage, length of spur, and strength to the wing
to strike in the spurred leg, in nearly the same manner as does the brutal
cockfighter by the careful selection of his best cocks. How low in the
scale of nature the 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
observed fighting all day long; male stag-beetles sometimes bear wounds
from the huge mandibles of other males; the males of certain hymenopterous
insects have been frequently seen by that inimitable observer M. Fabre,
fighting for a particular female who sits by, an apparently unconcerned
beholder of the struggle, and then retires with the conqueror. 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 of the
lion, and the hooked jaw to the male salmon; for the shield may be as
important for victory as the sword or spear.
Among 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 with the most elaborate care, and
show off in the best manner, their gorgeous plumage; they likewise 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 a pied
peacock was eminently attractive to all his hen birds. I cannot here enter
on the necessary details; but if man can in a short time give beauty and an
elegant carriage 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. Some
well-known laws, with respect to the plumage of male and female birds, in
comparison with the plumage of the young, can partly be explained through
the action of sexual selection on variations occurring at different ages,
and transmitted to the males alone or to both sexes at corresponding ages;
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, by individual males having had, in successive generations, some
slight advantage over other males, in their weapons, means of defence, or
charms; which they have transmitted to their male offspring alone. Yet, I
would not wish to attribute all sexual differences to this agency: for we
see in our domestic animals peculiarities arising and becoming attached to
the male sex, which apparently have not been augmented through selection by
man. The tuft of hair on the breast of the wild turkey-cock cannot be of
any use, and it is doubtful whether it can be ornamental in the eyes of the
female bird; indeed, had the tuft appeared under domestication it would
have been called a monstrosity.
ILLUSTRATIONS OF THE ACTION OF NATURAL SELECTION, OR THE SURVIVAL OF THE
FITTEST.
In order to make it clear how, as I believe, natural selection acts, I must
beg permission to give one or two imaginary illustrations. Let us take the
case of a wolf, which preys on various animals, securing some by craft,
some by strength, and some by fleetness; and let us suppose that the
fleetest prey, a deer for instance, had from any change in the country
increased in numbers, or that other prey had decreased in numbers, during
that season of the year when the wolf was hardest pressed for food. Under
such circumstances the swiftest and slimmest wolves have the best chance of
surviving, and so be preserved or selected, provided always that they
retained strength to master their prey at this or some other period of the
year, when they were compelled to prey on other animals. I can see no more
reason to doubt that this would be the result, than that man should be able
to improve the fleetness of his greyhounds by careful and methodical
selection, or by that kind of unconscious selection which follows from each
man trying to keep the best dogs without any thought of modifying the
breed. I may add that, according to Mr. Pierce, there are two varieties of
the wolf inhabiting the Catskill Mountains, in the United States, one with
a light greyhound-like form, which pursues deer, and the other more bulky,
with shorter legs, which more frequently attacks the shepherd's flocks.
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.
It should be observed that in the above illustration, I speak of the
slimmest individual wolves, and not of any single strongly marked variation
having been preserved. In former editions of this work I sometimes spoke
as if this latter alternative had frequently occurred. I saw the great
importance of individual differences, and this led me fully to discuss the
results of unconscious selection by man, which depends on the preservation
of all the more or less valuable individuals, and on the destruction of the
worst. I saw, also, that the preservation in a state of nature of any
occasional deviation of structure, such as a monstrosity, would be a rare
event; and that, if at first preserved, it would generally be lost by
subsequent intercrossing with ordinary individuals. Nevertheless, until
reading an able and valuable article in the "North British Review" (1867),
I did not appreciate how rarely single variations, whether slight or
strongly marked, could be perpetuated. The author takes the case of a pair
of animals, producing during their lifetime two hundred offspring, of
which, from various causes of destruction, only two on an average survive
to pro-create their kind. This is rather an extreme estimate for most of
the higher animals, but by no means so for many of the lower organisms. He
then shows that if a single individual were born, which varied in some
manner, giving it twice as good a chance of life as that of the other
individuals, yet the chances would be strongly against its survival.
Supposing it to survive and to breed, and that half its young inherited the
favourable variation; still, as the Reviewer goes onto show, the young
would have only a slightly better chance of surviving and breeding; and
this chance would go on decreasing in the succeeding generations. The
justice of these remarks cannot, I think, be disputed. If, for instance, a
bird of some kind could procure its food more easily by having its beak
curved, and if one were born with its beak strongly curved, and which
consequently flourished, nevertheless there would be a very poor chance of
this one individual perpetuating its kind to the exclusion of the common
form; but there can hardly be a doubt, judging by what we see taking place
under domestication, that this result would follow from the preservation
during many generations of a large number of individuals with more or less
strongly curved beaks, and from the destruction of a still larger number
with the straightest beaks.
It should not, however, be overlooked that certain rather strongly marked
variations, which no one would rank as mere individual differences,
frequently recur owing to a similar organisation being similarly acted on--
of which fact numerous instances could be given with our domestic
productions. In such cases, if the varying individual did not actually
transmit to its offspring its newly-acquired character, it would
undoubtedly transmit to them, as long as the existing conditions remained
the same, a still stronger tendency to vary in the same manner. There can
also be little doubt that the tendency to vary in the same manner has often
been so strong that all the individuals of the same species have been
similarly modified without the aid of any form of selection. Or only a
third, fifth, or tenth part of the individuals may have been thus affected,
of which fact several instances could be given. Thus Graba estimates that
about one-fifth of the guillemots in the Faroe Islands consist of a variety
so well marked, that it was formerly ranked as a distinct species under the
name of Uria lacrymans. In cases of this kind, if the variation were of a
beneficial nature, the original form would soon be supplanted by the
modified form, through the survival of the fittest.
To the effects of intercrossing in eliminating variations of all kinds, I
shall have to recur; but it may be here remarked that most animals and
plants keep to their proper homes, and do not needlessly wander about; we
see this even with migratory birds, which almost always return to the same
spot. Consequently each newly-formed variety would generally be at first
local, as seems to be the common rule with varieties in a state of nature;
so that similarly modified individuals would soon exist in a small body
together, and would often breed together. If the new variety were
successful in its battle for life, it would slowly spread from a central
district, competing with and conquering the unchanged individuals on the
margins of an ever-increasing circle.
It may be worth while to give another and more complex illustration of the
action of natural selection. Certain plants excrete sweet juice,
apparently for the sake of eliminating something injurious from the sap:
this is effected, for instance, by glands at the base of the stipules in
some Leguminosae, and at the backs of the leaves of the common laurel.
This juice, though small in quantity, is greedily sought by insects; but
their visits do not in any way benefit the plant. Now, let us suppose that
the juice or nectar was excreted from the inside of the flowers of a
certain number of plants of any species. Insects in seeking the nectar
would get dusted with pollen, and would often transport it from one flower
to another. The flowers of two distinct individuals of the same species
would thus get crossed; and the act of crossing, as can be fully proved,
gives rise to vigorous seedlings, which consequently would have the best
chance of flourishing and surviving. The plants which produced flowers
with the largest glands or nectaries, excreting most nectar, would oftenest
be visited by insects, and would oftenest be crossed; and so in the
long-run would gain the upper hand and form a local variety. The flowers,
also, which had their stamens and pistils placed, in relation to the size
and habits of the particular insect which visited them, so as to favour in
any degree the transportal of the pollen, would likewise be favoured. We
might have taken the case of insects visiting flowers for the sake of
collecting pollen instead of nectar; and as pollen is formed for the sole
purpose of fertilisation, its destruction appears to be a simple loss to
the plant; yet if a little pollen were carried, at first occasionally and
then habitually, by the pollen-devouring insects from flower to flower, and
a cross thus effected, although nine-tenths of the pollen were destroyed it
might still be a great gain to the plant to be thus robbed; and the
individuals which produced more and more pollen, and had larger anthers,
would be selected.
When our plant, by the above process long continued, had been rendered
highly attractive to insects, they would, unintentionally on their part,
regularly carry pollen from flower to flower; and that they do this
effectually I could easily show by many striking facts. I will give only
one, as likewise illustrating one step in the separation of the sexes of
plants. Some holly-trees bear only male flowers, which have four stamens
producing a rather 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 a
few pollen-grains, and on some a profusion. 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, which had 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 might be effected. It would
take up too much space to show the various steps, through dimorphism and
other means, by which the separation of the sexes in plants of various
kinds is apparently now in progress; but I may add that some of the species
of holly in North America are, according to Asa Gray, in an exactly
intermediate condition, or, as he expresses it, are more or less
dioeciously polygamous.
Let us now turn to the nectar-feeding insects; 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 with a very little more
trouble they can enter by the mouth. Bearing such facts in mind, it may be
believed that under certain circumstances individual differences in the
curvature or length of the proboscis, etc., too slight to be appreciated by
us, might profit a bee or other insect, so that certain individuals would
be able to obtain their food more quickly than others; and thus the
communities to which they belonged would flourish and throw off many swarms
inheriting the same peculiarities. The tubes of the corolla of the common
red or 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. That this nectar is much liked by the hive-bee is certain; for I
have repeatedly seen, but only in the autumn, many hive-bees sucking the
flowers through holes bitten in the base of the tube by humble bees. The
difference in the length of the corolla in the two kinds of clover, which
determines the visits of the hive-bee, must be very trifling; for I have
been assured that when red clover has been mown, the flowers of the second
crop are somewhat smaller, and that these are visited by many hive-bees. I
do not know whether this statement is accurate; nor whether another
published statement can be trusted, namely, that the Ligurian bee, which is
generally considered a mere variety of the common hive-bee, and which
freely crosses with it, is able to reach and suck the nectar of the red
clover. Thus, in a country where this kind of clover abounded, it might be
a great advantage to the hive-bee to have a slightly longer or differently
constructed proboscis. On the other hand, as the fertility of this clover
absolutely depends on bees visiting the flowers, if humble-bees were to
become rare in any country, it might be a great advantage to the plant to
have a shorter or more deeply divided corolla, so that the hive-bees should
be enabled to suck 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 to each other in the most perfect manner, by the
continued preservation of all the individuals which presented slight
deviations of structure mutually favourable to each other.
I am well aware that this doctrine of natural selection, exemplified in the
above imaginary instances, is open to the same objections which were first
urged against Sir Charles Lyell's noble views on "the modern changes of the
earth, as illustrative of geology;" but we now seldom hear the agencies
which we see still at work, spoken of as trifling and insignificant, when
used in explaining the excavation of the deepest valleys or the formation
of long lines of inland cliffs. Natural selection acts only by the
preservation and accumulation of 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 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 (with the exception of the curious and not well understood
cases of parthenogenesis) unite for each birth; but in the case of
hermaphrodites this is far from obvious. Nevertheless there is reason to
believe that with all hermaphrodites two individuals, either occasionally
or habitually, concur for the reproduction of their kind. This view was
long ago doubtfully suggested by Sprengel, Knight and Kolreuter. We shall
presently see its importance; but I must here treat the subject with
extreme brevity, though I have the materials prepared for an ample
discussion. All vertebrate animals, all insects and some other large
groups of animals, pair for each birth. Modern research has much
diminished the number of supposed hermaphrodites and of real hermaphrodites
a large number pair; that is, two individuals regularly unite for
reproduction, which is all that concerns us. But still there are many
hermaphrodite animals which certainly do not habitually pair, and a vast
majority of plants are hermaphrodites. What reason, it may be asked, is
there for supposing in these cases that two individuals ever concur in
reproduction? As it is impossible here to enter on details, I must trust
to some general considerations alone.
In the first place, I have collected so large a body of facts, and made so
many experiments, showing, in accordance with the almost universal belief
of breeders, that with animals and plants a cross between different
varieties, or between individuals of the same variety but of another
strain, gives 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 that no
organic being fertilises itself for a perpetuity of generations; but that a
cross with another individual is occasionally--perhaps at long intervals of
time--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! If an
occasional cross be indispensable, notwithstanding that the plant's own
anthers and pistil stand so near each other as almost to ensure self-
fertilisation, the fullest freedom for the entrance of pollen from another
individual will explain the above state of exposure of the organs. Many
flowers, on the other hand, have their organs of fructification closely
enclosed, as in the great papilionaceous or pea-family; but these almost
invariably present beautiful and curious adaptations in relation to the
visits of insects. So necessary are the visits of bees to many
papilionaceous flowers, that their fertility is greatly diminished if these
visits be prevented. Now, it is scarcely possible for insects to fly from
flower to flower, and not to carry pollen from one to the other, to the
great good of the plant. Insects act like a camel-hair pencil, and it is
sufficient, to ensure fertilisation, just to touch with the same brush the
anthers of one flower and then the stigma of another; but it must not be
supposed that bees would thus produce a multitude of hybrids between
distinct species; for if a plant's own pollen and that from another species
are placed on the same stigma, the former is so prepotent that it
invariably and completely destroys, as has been shown by Gartner, the
influence of 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 Kolreuter has shown to be the case with the barberry; and in
this very genus, which seems to have a special contrivance for
self-fertilisation, it is well known that, if 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 numerous other cases,
far from self-fertilisation being favoured, there are special contrivances
which effectually prevent the stigma receiving pollen from its own flower,
as I could show from the works of Sprengel and others, as well as from my
own observations: for instance, in Lobelia fulgens, there is a really
beautiful and elaborate contrivance by which all 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 raise plenty of seedlings. Another species of Lobelia, which
is visited by bees, seeds freely in my garden. In very many other cases,
though there is no special mechanical contrivance to prevent the stigma
receiving pollen from the same flower, yet, as Sprengel, and more recently
Hildebrand and others have 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 so-named
dichogamous plants have in fact separated sexes, and must habitually be
crossed. So it is with the reciprocally dimorphic and trimorphic plants
previously alluded to. 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 be in so
many cases 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 of the
seedlings thus raised turn out, as I found, 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; and the pollen of each flower
readily gets on its stigma without insect agency; for I have found that
plants carefully protected from insects produce the full number of pods.
How, then, comes it that such a vast number of the seedlings are
mongrelized? It must arise from the pollen of a distinct VARIETY having a
prepotent effect over the 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 reversed, 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 large 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 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, 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 informs me that
the rule does not hold good in Australia: but if most of the Australian
trees are dichogamous, the same result would follow as if they bore flowers
with separated sexes. I have made these few remarks on trees simply to
call attention to the subject.
Turning for a brief space to animals: various terrestrial species are
hermaphrodites, such as the land-mollusca and earth-worms; but these all
pair. As yet I have not found a single terrestrial animal which can
fertilise itself. This remarkable fact, which offers so strong a contrast
with terrestrial plants, is intelligible on the view of an occasional cross
being indispensable; for owing to the nature of the fertilising element
there are no means, analogous to the action of insects and of the wind with
plants, by which an occasional cross could be effected with terrestrial
animals without the concurrence of two individuals. Of aquatic animals,
there are many self-fertilising hermaphrodites; but here the currents of
water offer an obvious means for an occasional cross. As in the case of
flowers, I have as yet failed, after consultation with one of the highest
authorities, namely, Professor Huxley, to discover a single hermaphrodite
animal with the organs of reproduction so perfectly enclosed 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, under this point of view, a case of great difficulty; but I have
been enabled, by a fortunate chance, to prove that two individuals, though
both are self-fertilising hermaphrodites, do sometimes cross.
It must have struck most naturalists as a strange anomaly that, both with
animals and plants, some species of the same family and even of the same
genus, though agreeing closely with each other in their whole organisation,
are hermaphrodites, and some unisexual. But if, in fact, all
hermaphrodites do occasionally intercross, the difference between them and
unisexual species is, as far as function is concerned, very small.
>From these several considerations and from the many special facts which I
have collected, but which I am unable here to give, it appears that with
animals and plants an occasional intercross between distinct individuals is
a very general, if not universal, law of nature.
CIRCUMSTANCES FAVOURABLE FOR THE PRODUCTION OF NEW FORMS THROUGH NATURAL
SELECTION.
This is an extremely intricate subject. A great amount of variability,
under which term individual differences are always included, will evidently
be favourable. A large number of individuals, by giving a better chance
within any given period for the appearance of profitable variations, will
compensate for a lesser amount of variability in each individual, and is, I
believe, a highly important element of success. Though nature grants long
periods of time for the work of natural selection, she does not grant an
indefinite period; for as all organic beings are striving 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 be
exterminated. Unless favourable variations be inherited by some at least
of the offspring, nothing can be effected by natural selection. The
tendency to reversion may often check or prevent the work; but as this
tendency has not prevented man from forming by selection numerous domestic
races, why should it prevail against natural selection?
In the case of methodical selection, a breeder selects for some definite
object, and if the individuals be allowed freely to intercross, his work
will completely fail. But when many men, without intending to alter the
breed, have a nearly common standard of perfection, and all try to procure
and breed from the best animals, improvement surely but slowly follows from
this unconscious process of selection, notwithstanding that there is no
separation of selected individuals. Thus it will be under nature; for
within a confined area, with some place in the natural polity not perfectly
occupied, all the individuals varying in the right direction, though in
different degrees, will tend to be preserved. But if the area be large,
its several districts will almost certainly present different conditions of
life; and then, if the same species undergoes modification in different
districts, the newly formed varieties will intercross on the confines of
each. But we shall see in the sixth chapter that intermediate varieties,
inhabiting intermediate districts, will in the long run generally be
supplanted by one of the adjoining varieties. Intercrossing will chiefly
affect those animals which unite for each birth and wander much, and which
do not breed at a very quick rate. Hence with animals of this nature, for
instance birds, varieties will generally be confined to separated
countries; and this I find to be the case. With hermaphrodite organisms
which cross only occasionally, and likewise with animals which unite for
each birth, but which wander little and can increase at a rapid rate, a new
and improved variety might be quickly formed on any one spot, and might
there maintain itself in a body and afterward spread, so that the
individuals of the new variety would chiefly cross together. On this
principle nurserymen always prefer saving seed from a large body of plants,
as the chance of intercrossing is thus lessened.
Even with animals which unite for each birth, and which do not propagate
rapidly, we must not assume that free intercrossing would always eliminate
the effects of natural selection; for I can bring forward a considerable
body of facts showing that within the same area two varieties of the same
animal may long remain distinct, from haunting different stations, from
breeding at slightly different seasons, or from the individuals of each
variety preferring to pair together.
Intercrossing plays a very important part in nature by 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, as already stated, we have reason
to believe that occasional intercrosses take place with all animals and
plants. Even if these take place only at long intervals of time, 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 crosses, even at rare intervals, will be great. With respect
to organic beings extremely low in the scale, which do not propagate
sexually, nor conjugate, and which cannot possibly intercross, uniformity
of character can be retained by them under the same conditions of life,
only through the principle of inheritance, and through natural selection
which will destroy any individuals departing from the proper type. If the
conditions of life change and the form undergoes modification, uniformity
of character can be given to the modified offspring, solely by natural
selection preserving similar favourable variations.
Isolation also is an important element in the modification of species
through natural selection. In a confined or isolated area, if not very
large, the organic and inorganic conditions of life will generally be
almost uniform; so that natural selection will tend to modify all the
varying individuals of the same species in the same manner. Intercrossing
with the inhabitants of the surrounding districts, will also be thus
prevented. Moritz Wagner has lately published an interesting essay on this
subject, and has shown that the service rendered by isolation in preventing
crosses between newly-formed varieties is probably greater even than I
supposed. But from reasons already assigned I can by no means agree with
this naturalist, that migration and isolation are necessary elements for
the formation of new species. The importance of isolation is likewise
great in preventing, after any physical change in the conditions, such as
of climate, elevation of the land, etc., the immigration of better adapted
organisms; and thus new places in the natural economy of the district will
be left open to be filled up by the modification of the old inhabitants.
Lastly, isolation will give time for a new variety to be improved at a slow
rate; and this may sometimes be of much importance. 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
inhabitants will be small; and this will retard the production of new
species through natural selection, by decreasing the chances of favourable
variations arising.
The mere lapse of time by itself does nothing, either for or against
natural selection. I state this because it has been erroneously asserted
that the element of time has been assumed by me to play an all-important
part in modifying species, as if all the forms of life were necessarily
undergoing change through some innate law. Lapse of time is only so far
important, and its importance in this respect is great, that it gives a
better chance of beneficial variations arising and of their being selected,
accumulated, and fixed. It likewise tends to increase the direct action of
the physical conditions of life, in relation to the constitution of each
organism.
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 number of the
species inhabiting it is 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 in the world.
Hence an oceanic island at first sight seems to have been highly favourable
for the production of new species. But we may thus 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 isolation is of great importance in the production of new species,
on the whole I am inclined to believe that largeness of area is still more
important, especially for the production of species which shall 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 much more 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
other forms. Moreover, great areas, though now continuous, will often,
owing to former oscillations of level, have 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
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, and will give rise
to the greatest number of new varieties and species. They will thus play a
more important part in the changing history of the organic world.
In accordance with this view, we can, perhaps, understand some facts which
will be again alluded to in our chapter on Geographical Distribution; for
instance, the fact of the productions of the smaller continent of Australia
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, we can understand how it is that the flora of
Madeira, according to Oswald Heer, resembles to a certain extent 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.
Consequently, the competition between fresh water productions will have
been less severe than elsewhere; new forms will have been more slowly
produced, and old forms more slowly exterminated. And it is in fresh water
basins 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
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