NATURAL SELECTION BY THE STRUGGLE FOR EXISTENCE. DIVISION OF LABOUR AND
PROGRESS.
Interaction of the Two Organic Formative Causes,
Inheritance and Adaptation.—Natural and Artificial
Selection.—Struggle for Existence, or Competition for
the Necessaries of Life.—Disproportion between the
Number of Possible or Potential, and the Number of Real
or Actual Individuals.—Complicated Correlations of all
Neighbouring Organisms.—Mode of Action in Natural
Selection.—Homochromic Selection as the Cause of
Sympathetic Colourings.—Sexual Selection as the Cause of
the Secondary Sexual Characters.—Law of Separation or
Division of Labour (Polymorphism, Differentiation,
Divergence of Characters).—Transition of Varieties into
Species.—Idea of Species.—Hybridism.—Law of Progress
or Perfectioning (Progressus, Teleosis).
In order to arrive at a right understanding of Darwinism, it is, above
all, necessary that the two organic functions of _Inheritance and
Adaptation_, which we spoke of in our last chapter, should be more
closely examined. If we do not, on the one hand, examine the purely
mechanical nature of these two physiological activities, and the various
action of their different laws, and if, on the other hand, we do not
consider how complicated the interaction of these different laws of
Inheritance and Adaptation must be, we shall not be able to understand
how these two functions, by themselves, have been able to produce all
the variety of animal and vegetable forms, which, in fact, they have.
We have, at least, hitherto been unable to discover any other formative
causes besides these two, and if we rightly understand the necessary and
infinitely complicated interaction of Inheritance and Adaptation, we do
not require to look for other unknown causes for the change of organic
forms. These two fundamental causes are, as far as we can see,
completely sufficient.
Even long before Darwin had published his Theory of Selection, some
naturalists, and especially Goethe, had assumed the interaction of two
distinct formative tendencies—a conservative or preserving, and a
progressive or changing formative tendency—as the causes of the variety
of organic forms. The former was called by Goethe the centripetal or
specifying tendency, the latter the centrifugal tendency, or the
tendency to metamorphosis (p. 89). These two tendencies completely
correspond with the two processes of Inheritance and Adaptation.
_Inheritance_ is the _centripetal_ or _internal formative tendency_
which strives to keep the organic form in its species, to form the
descendants like the parents, and always to produce identical things
from generation to generation. _Adaptation_, on the other hand, which
counteracts inheritance, is the _centrifugal_ or _external formative
tendency_, which constantly strives to change the organic forms through
the influence of the varying agencies of the outer world, to create new
forms out of those existing, and entirely to destroy the constancy or
permanency of species. Accordingly as Inheritance or Adaptation
predominates in the struggle, the specific form either remains constant
or changes into a new species. The degree of constancy of form in the
different species of animals and plants, which obtains at any moment,
is simply the necessary result of the momentary predominance which
either of these two formative powers (or physiological activities) has
acquired over the other.
If we now return to the consideration of the process of selection or
choice, the outlines of which we have already examined, we shall be in a
position to see clearly and distinctly that both artificial and natural
selection rest solely upon the interaction of these two formative
tendencies. If we carefully watch the proceedings of an artificial
selector—a farmer or a gardener—we find that only these two
constructive forces are used by him for the production of new forms. The
whole art of artificial selection rests solely upon a thoughtful and
wise application of the laws of Inheritance and Adaptation, and upon
their being applied and regulated in an artistic and systematic manner.
Here the will of man constitutes the selecting force.
The case of natural selection is quite similar, for it also employs
merely these two organic constructive forces, these ingrained
physiological properties of Adaptation and Heredity, in order to produce
the different species. But the selecting principle or force, which in
_artificial_ selection is represented by the conscious _will of man_
acting for a definite purpose, consists in _natural_ selection of the
unconscious _struggle for existence_ acting without a definite plan.
What we mean by “struggle for existence” has already been explained in
the seventh chapter. It is the recognition of this exceedingly important
identity which constitutes one of the greatest of Darwin’s merits. But
as this relation is very frequently imperfectly or falsely understood,
it is necessary to examine it now more closely, and to illustrate by a
few examples the operation of the struggle for life, and the operation
of natural selection _by means of_ the struggle for life (Gen. Morph.
ii. 231).
When considering the struggle for life, we started from the fact that
the number of germs which all animals and plants produce is infinitely
greater than the number of individuals which actually come to life and
remain alive for a longer or shorter time. Most organisms produce during
life thousands or millions of germs, from each of which, under
favourable circumstances, a new individual might arise. In most animals
and plants these germs are eggs, that is cells, which for their
development require sexual fructification. But among the Protista, the
lowest organisms, which are neither animals nor plants, and which
propagate themselves only in a non-sexual manner, the germ-cells, or
spores, require no fructification. Now, in all cases the number of
unsexual, as well as of sexual germs, is out of all proportion to the
number of actually living individuals of every species.
Taken as a whole, the number of living animals and plants on our earth
remains always about the same. The number of places in the economy of
nature is limited, and in most parts of the earth’s surface these places
are always approximately occupied. Certainly there occur everywhere and
in every year fluctuations in the absolute and in the relative number of
individuals of all species. However, taken as a whole, these
fluctuations are of little importance, and it is broadly the fact that
the total number of all individuals remains, on an average, almost
constant. There is a constant fluctuation, which depends on the fact
that in one year or another one or other series of animals and plants
predominates, and that every year the struggle for life somewhat alters
their relations.
Every single species of animals and plants would have densely peopled
the whole earth’s surface in a short time, if it had not had to struggle
against a number of enemies and hostile influences. Even Linnæus
calculated that if an annual plant only produced two seeds (and there is
not one which produces so few), it would have yielded in twenty years a
million of individuals. Darwin has calculated of elephants, which of all
animals seem the slowest to increase, that in seven hundred and fifty
years the descendants of a single pair would amount to nineteen millions
of individuals; this is supposing that every elephant, during its period
of fertility (from the 30th to the 90th year), produced only three pairs
of young ones, and survived itself to its hundredth year. In like manner
the increase of the number of human beings—if calculated on the average
proportion of births to population, and no hindrances to the natural
increase stood in the way—would be such as to double the total in
twenty-five years. In every century the total number of men would have
increased sixteen-fold; whereas we know that the total number of human
beings increases but slowly, and that the increase of population is very
different in different countries. While European tribes spread over the
whole globe, other tribes or species of men every year draw nearer to
their complete extinction. This is the case especially with the redskins
of America, and with the copper-coloured natives of Australia. Even if
these races were to propagate more abundantly than the white Europeans,
yet they would sooner or later succumb to the latter in the struggle for
life. But of all human individuals, as of all other organisms, by far
the majority perish at the earliest period of their lives. Of the
immense quantity of germs which every species produce, only very few
actually succeed in developing, and of these few it is again only a very
small portion which attain to the age in which they can reproduce
themselves (compare p. 161).
From the disproportion between the immense excess of organic germs and
the small number of chosen individuals which are actually able to
continue in existence beside one another, there follows of necessity
that universal struggle for life, that constant fight for existence,
that perpetual competition for the necessaries of life, of which I gave
a sketch in my seventh chapter. It is this struggle for life which
brings natural selection into play, which in its turn is made use of by
the interaction of the phenomena of Inheritance and Adaptation as a
sifting agency, and which thus causes a continual change in all organic
forms. In this struggle for acquiring the necessary conditions of
existence, those individuals will always overpower their rivals who
possess any individual privilege, any advantageous quality, of which
their fellow competitors are destitute. It is true we are able only in
the fewest cases (in those animals and plants best known to us) to form
an approximate conception of the infinitely complicated interaction of
the numerous circumstances, all of which here come into combination.
Only think how infinitely varied and complicated are the relations of
every single human being to the rest of mankind, and in general, to the
whole of the surrounding outer world. But similar relations prevail also
among all animals and plants which live together in one place. All
influence one another actively or passively. Every animal and every
plant struggles directly with a number of enemies, beasts of prey,
parasitic animals, etc. Plants standing together struggle with one
another for the space of ground requisite for their roots, for the
necessary amount of light, air, moisture, etc. In like-manner, animals
living together struggle with one another for their food,
dwelling-place, etc. In this most active and complicated struggle, any
personal superiority, however small, any individual advantage, may
possibly decide the issue in favour of the one possessing it. This
privileged individual remains the victor in the struggle, and propagates
itself, while its fellow-competitors perish before they succeed in
propagating themselves. The personal advantage which gave it the victory
is transmitted by inheritance to its descendants, and by a further
development may become so strongly marked as to cause us to consider the
later generations as a new species.
The infinitely complicated correlations which exist between the
organisms of every district, and which must be looked upon as the real
conditions of the struggle for life, are mostly unknown to us, and are
very difficult to discover. We have hitherto been able to trace them
only to a certain point in individual cases, as in the example given by
Darwin of the relations between cats and red clover in England. The red
clover (_Trifolium pratense_), which in England is among the best fodder
for cattle, requires the visit of humming-bees in order to attain the
formation of seeds. These insects, while sucking the honey from the
bottom of the flower, bring the pollen in contact with the stigma, and
thus cause the fructification of the flower, which never takes place
without it. Darwin has shown by experiments, that red clover which is
not visited by humming-bees does not yield a single seed. The number of
bees is determined by the number of their enemies, the most destructive
of which are the field-mice. The more the field-mice predominate, the
less the clover is fructified. The number of field-mice, again, is
dependent upon the number of their enemies, principally cats. Hence in
the neighbourhood of villages and towns, where many cats are kept, there
are plenty of bees. A great number of cats, therefore, is evidently of
great advantage for the fructification of clover. This example may be
followed still further, as has been done by Carl Vogt, if we consider
that cattle which feed on red clover are one of the most important
foundations of the wealth of England. Englishmen preserve their bodily
and mental powers chiefly by making excellent meat—roast beef and
beefsteak—their principal food. The English owe the superiority of
their brains and minds over those of other nations in a great measure to
their excellent meat. But this is clearly indirectly dependent upon the
cats, which pursue the mice. We may, with Huxley, even trace the chain
of causes to those old maids who cherish and keep cats, and,
consequently, are of the greatest importance to the fructification of
the clover and to the prosperity of England. From this example we can
see that the further it is traced the wider is the circle of action and
of correlation. We can with certainty maintain that there exist a great
number of such correlations in every plant and in every animal, only we
are not always able to point out and survey their concatenation as in
the last instance.
Another remarkable example of important correlations is the following,
given by Darwin. In Paraguay, there are no wild oxen and horses, as in
the neighbouring parts of South America, both north and south of
Paraguay. This surprising circumstance is explained simply by the fact
that in that country a kind of small fly is very frequent, and is in the
habit of laying its eggs in the navel of newly-born calves and foals.
The newly-born animals die in consequence of this attack, and the small
deadly fly is therefore the cause of oxen and horses never becoming wild
in that district. Supposing that this fly were destroyed by some
insect-eating bird, then these large mammals would grow wild in
Paraguay, as well as in the neighbouring parts of South America; and as
they would eat a quantity of certain species of plants, the whole flora,
and, consequently again, the whole fauna of the country would become
changed. It is hardly necessary to state, that at the same time the
whole economy, and consequently the character, of the human population
would alter.
Thus the prosperity, nay, even the existence of whole populations can be
indirectly determined by a single small animal or vegetable form in
itself extremely insignificant. There are small coral islands whose
human inhabitants live almost entirely upon the fruit of a species of
palm. The fructification of this palm is principally effected by
insects, which carry the pollen from the male to the female palm trees.
The existence of these useful insects is endangered by insect-eating
birds, which in their turn are pursued by birds of prey. The birds of
prey, however, often succumb to the attack of a small parasitical mite,
which develops itself in millions in their feathers. This small,
dangerous parasite, again, may be killed by parasitical moulds. Moulds,
birds of prey, and insects would in this case favour the prosperity of
the palm, and consequently of man; birds, mites, and insect-eating birds
would, on the other hand, endanger it.
Interesting examples in relation to the change of correlations in the
struggle for life are furnished also by those isolated oceanic islands,
uninhabited by man, on which at different times goats and pigs have been
placed by navigators. These animals become wild, and having no enemies,
they increase in number so excessively, that the rest of the animal and
vegetable population suffer in consequence, and the island finally may
become almost a waste, because there is insufficient food for the large
mammals which increase too numerously. In some cases on an island thus
overrun with goats and pigs, other navigators have let loose a couple of
dogs, who enjoyed this superabundance of food, and they again increased
so numerously, and made such havoc among the herds, that after several
years the dogs themselves lacked food, and they also almost died out.
The equilibrium of species continually changes in this manner in
nature’s economy, accordingly as one or another species increases at the
expense of the rest. In most cases the relations of different species of
animals and plants to one another are much too complicated for us to be
able to follow them, and I leave it to the reader to picture to himself
what an infinitely complicated machinery is at work in every part of the
world in consequence of this struggle. The impulses which started the
struggle, and which altered and modified it in different places, are in
the end seen to be the impulses of self-preservation—in fact, the
instinct leading individuals to preserve themselves (the instinct of
obtaining food), and the instinct leading them to preserve the species
(instinct of propagation). It is these two fundamental instincts of
organic self-preservation of which Schiller, the idealist (not Goethe,
the realist!) says:
“Meanwhile, until philosophy
Sustains the structure of the world,
Her workings will be carried on
By hunger and by love.”[4]
It is these two powerful fundamental instincts which, by their varying
activity, produce such extraordinary differences in species through the
struggle for life. They are the foundations of the phenomena of
Inheritance and Adaptation. We have, in fact, traced all phenomena of
Inheritance to propagation, all phenomena of Adaptation to nutrition, as
the two wider classes of material phenomena to which they belong.
The struggle for life in natural selection acts with as much selective
power as does the will of man in artificial selection. The latter,
however, acts according to a plan and consciously, the former without a
plan and unconsciously. This important difference between artificial and
natural selection deserves especial consideration. For we learn by it to
understand how _arrangements serving a purpose can be produced by
mechanical causes acting without an object, as well as by causes acting
for an object_. The products of natural selection are arranged even more
for a purpose than the artificial products of man, and yet they owe
their existence not to a creative power acting for a definite purpose,
but to a mechanical relation acting unconsciously and without a plan.
If we had not thoroughly considered the interaction of Inheritance and
Adaptation under the influence of the struggle for life, we should not
at first be inclined to expect such results from this natural process of
selection as are, in fact, furnished by it. It may therefore be
appropriate here to mention a few especially striking examples of the
activity of natural selection.
Let us first take _Darwin’s homochromic selection_ of animals, or the
so-called “sympathetic selection of colours,” into consideration.
Earlier naturalists have remarked that numerous animals are of nearly
the same colour as their dwelling-place, or the surroundings in which
they permanently live. Thus, for example, plant-lice and many other
insects living on leaves are of a green colour. The inhabitants of the
deserts, the jerboa, or leaping mice, foxes of the desert, gazelles,
lions, etc., are mostly of a yellow or yellowish-brown colour, like the
sand of the desert. The polar animals, which live on the ice and snow,
are white or grey, like ice and snow. Many of these animals change their
colour in summer and winter. In summer, when the snow partly vanishes,
the fur of these polar creatures becomes brownish-grey or blackish, like
the naked earth, while in winter it again becomes white. Butterflies and
insects which hover round the gay and bright flowers are like them in
colour. Now, Darwin explains this surprising circumstance quite simply
by the fact that such colours as agree with the colour of the habitation
are of the greatest use to the animals concerned. If these animals are
animals of prey, they will be able to approach the object of their
pursuit more safely and with less likelihood of observation, and, in
like manner, those animals which are pursued will be able to escape
more easily, if their colour is as little different as possible from
that of their surroundings. If therefore originally an animal species
varied so as to present cases of all colours, those individuals whose
colour most resembled the surroundings must have been most favoured in
the struggle for life. They remained more unobserved, maintained and
propagated themselves, while those individuals or varieties differently
coloured died out.
I have tried to explain, by the same sympathetic selection of colour,
the wonderful fact that the majority of pelagic animals—that is, of
those which live on the surface of the open sea—are bluish, or
completely colourless and transparent, like glass and water itself. Such
colourless, glassy animals are met with in the most different classes.
To them belong, among fish, the Helmicthyidæ, through whose crystalline
bodies the words of a book can be read; among the molluscs, the finned
snails (Heteropods) and sea-butterflies, or whales-food (Pteropods);
among worms, the Salpæ, Alciope, and Sagitta; further, a great number of
pelagic crabs (Crustacea), and the greater part of the Medusæ
Umbrella-jellies, (Discomedusæ); Comb-jellies, (Ctenophora). All of
these pelagic animals, which float on the surface of the ocean, are
transparent and colourless, like glass and like the water itself, while
their nearest kin live at the bottom of the ocean, and are coloured and
opaque like the inhabitants of the land. This remarkable fact, like the
sympathetic colouring of the inhabitants of the earth, can be explained
by natural selection. Among the ancestors of the pelagic glass-like
animals which showed a different degree of colourlessness and
transparency, those that were the most colourless and transparent must
have been most favoured in the active struggle for life which takes
place on the surface of the ocean. They were enabled to approach their
prey the most easily unobserved, and were themselves least observed by
their enemies. Hence they could preserve and propagate themselves more
easily than their more coloured and opaque relatives; and finally, by
accumulative adaptation and transmission by inheritance, through natural
selection, in the course of many generations their bodies would attain
that degree of crystal-like transparency and colourlessness which we at
present admire in them. (Gen. Morph. ii. 242.)
No less interesting and instructive than homochromic selection is that
species of natural selection which Darwin calls “_sexual selection_,”
which explains the origin of the so-called “secondary sexual
characters.” We have already mentioned these subordinate sexual
characteristics, so instructive in many respects. They comprise those
peculiarities of animals and plants which belong only to one of the two
sexes, and which do not stand in any direct relation to the act of
propagation itself (compare above, p. 244). Such secondary sexual
characters occur in great variety among animals. We all know how
striking is the difference of the two sexes in size and colour in many
birds and butterflies. The male sex is generally the larger and more
beautiful. It often possesses special decorations or weapons; as for
example, the spur and comb of the cock, the antlers of the stag and
deer, etc. All these peculiarities of the two sexes have nothing
directly to do with propagation itself, which is effected by the
“primary sexual characters,” or actual sexual organs.
Now, the origin of these remarkable “secondary sexual characters” is
explained by Darwin simply by a choice or selection which takes place
in the propagation of animals. In most animals the number of individuals
of both sexes is unequal; either the number of the female or the number
of the male individuals is greater, and, as a rule, when the season of
propagation approaches, a struggle takes place between the rivals for
the possession of the animals of the other sex. It is well known with
what vigour and vehemence this struggle is fought out among the higher
animals—among mammals and birds—especially among those of polygamous
habits. Among gallinaceous birds, where for one cock there are several
hens, a severe struggle takes place between the competing cocks for as
large a harem as possible. The same is the case with many ruminating
animals. Among stags and deer, for instance, at the period of rut,
deadly struggles take place between the males for the possession of the
females. The secondary sexual character which here distinguishes the
males—the antlers of stags and deer—not possessed by the female, is,
according to Darwin, the consequence of that struggle. Here the motive
and cause determining the struggle is not, as in the case of the
struggle for individual existence, self-preservation, but the
preservation of the species—propagation. There are numerous passive
weapons of defence, as well as active weapons for attack. The lion’s
mane, not possessed by the female, is evidently such a weapon of
defence; it is an excellent means of protection against the bites which
the male lions try to inflict on each other’s necks when fighting for
the females; consequently those males with the strongest manes have the
greatest advantage in the sexual struggle. The dewlap of the ox and the
comb of the cock are similar defensive weapons. Active weapons of
attack, on the other hand, are the antlers of the stag, the tusks of
the boar, the spur of the cock, and the hugely developed pair of jaws in
the male stag-beetle; all are instruments employed by the males in the
struggle for the females, for annihilating or chasing away their rivals.
In the cases just mentioned, it is the bodily “struggle to the death”
which determines the origin of the secondary sexual characters. But,
besides these mortal struggles, there are other important competitions
in sexual selection, which no less influence the structure of the
rivals. These consist principally in the fact that the courting sex
tries to please the other by external finery, by beauty of form, or by a
melodious voice. Darwin thinks that the beautiful voices of singing
birds have principally originated in this way. Many male birds carry on
a regular musical contest when they contend for the possession of the
females. It is known of several singing birds, that in the breeding
season the males assemble in numbers round the females, and let their
songs resound before them, and that then the females choose the singers
who best please them for their mates. Among other songsters, individual
males pour out their songs in the loneliness of the forest in order to
attract the females, and the latter follow the most attractive calls. A
similar musical contest, though certainly less melodious, takes place
among crickets and grasshoppers. The male cricket has on its belly two
instruments like drums, and produces with these the sharp chirping notes
which the ancient Greeks curiously enough thought beautiful music. Male
grasshoppers, partly by using their hind-legs like the bow of a violin
against their wing coverings, and partly by rubbing their wing coverings
together, bring out tones which are, indeed, not melodious to us, but
which please the female grasshoppers so much that they choose the male
who fiddles the best.
Among other insects and birds it is not song or, in fact, any musical
accomplishment, but finery or beauty of the one sex which attracts the
other. Thus we find that, among most gallinaceous birds, the cocks are
distinguished by combs on their heads, or by a beautiful tail, which
they can spread out like a fan; as for example, in the case of the
peacock and turkey-cock. The magnificent tail of the bird of paradise is
also an exclusive ornament of the male sex. In like manner, among very
many other birds and very many insects, principally among butterflies,
the males are distinguished from the females by special colours or other
decorations. These are evidently the results of sexual selection. As the
females do not possess these attractions and decorations, we must come
to the conclusion that they have been acquired by degrees by the males
in the competition for the females, which takes its origin in the
selective discrimination of the females.
We may easily picture to ourselves, in detail, the application of this
interesting conclusion to the human community. Here, also, the same
causes have evidently influenced the development of the secondary sexual
characters. The characteristics distinguishing the man, as well as those
distinguishing the woman, owe their origin, certainly for the most part,
to the sexual selection of the other sex. In antiquity and in the Middle
Ages, especially in the romantic age of chivalry, it was the bodily
struggles to the death—the tournaments and duels—which determined the
choice of the bride; the strongest carried home the bride. In more
recent times, however, in our so-called “polished” or “highly
civilized” society, competing rivals prefer to contend indirectly by
means of musical accomplishments, instrumental performances and song, by
bodily charms, natural beauty, or artificial decoration. But by far the
most important of these different forms of sexual selection in man is
that form which is the most exalted, namely, _psychical selection_, in
which the mental excellencies of the one sex influence and determine the
choice of the other. The most highly intellectually developed types of
men have, throughout generations, when choosing a partner in life, been
guided by her excellencies of soul, and have thus transmitted these
qualities to their posterity, and they have in this way, more than by
any other thing, helped to create the deep chasm which at present
separates civilized men from the rudest savages, and from our common
animal ancestors. In fact, both the part played by the prevalence of a
higher standard of sexual selection, and the part played by the due
division of labour between the two sexes, is exceedingly important, and
I believe that here we must seek for the most powerful causes which have
determined the origin and the historical development of the races of
man. (Gen. Morph. ii. 247.) As Darwin, in his exceedingly interesting
work, published in 1871, on “The Origin of Man and Sexual
Selection,”(48) has discussed this subject in the most masterly manner,
and has illustrated it by most remarkable examples, I refer for further
detail to that work.
But now let us look again at two extremely important organic laws which
can be explained by the theory of selection, as necessary consequences
of natural selection in the struggle for existence. I mean the law of
_division of labour_, or _differentiation_, and the law of _progress_,
or _perfecting_. When the phenomena due to these two laws first became
known, through observation of the historical development, the individual
development, and the comparative anatomy of animals and plants,
naturalists were inclined to trace them to a direct creative influence.
It was supposed to be part of the plan of the Creator, acting for a
definite purpose, in the course of time to develop the forms of animals
and plants more and more variously, and to bring them more and more to a
state of perfection. We shall evidently make a great advance in the
knowledge of nature if we reject this teleological and anthropomorphic
conception, and if we can prove the two laws of Division of Labour and
Perfecting to be the necessary consequences of natural selection in the
struggle for life.
The first great law which follows directly and of necessity from natural
selection, is that of _separation_, or _differentiation_, which is
frequently called _division of labour_, or _polymorphism_, and which
Darwin speaks of as _divergence of character_. (Gen. Morph. ii. 249.) We
understand by it the general tendency of all organic individuals to
develop themselves more and more diversely, and to deviate from the
common primary type. The cause of this general inclination towards
differentiation and the formation of heterogeneous forms from
homogeneous beginnings is, according to Darwin, simply to be traced to
the circumstance that the struggle for life between every two organisms
rages all the more fiercely the nearer the relation in which they stand
to one another, or the more nearly alike they are. This is an
exceedingly important, and in reality an exceedingly simple relation,
but it is usually not duly considered.
It must be obvious to every one, that in a field of a certain size,
beside the corn-plants which have been sown, a great number of weeds
can exist, and, moreover, in places which could not have been occupied
by corn-plants. The more dry and sterile places of the ground, in which
no corn-plant would thrive, may still furnish sustenance to weeds of
different kinds; and such species and individuals of weeds will more
readily be able to exist in such conditions, in proportion as they are
suited to adapt themselves to the different parts of the ground. It is
the same with animals. It is evident that a much greater number of
animal individuals can live together in one and the same limited
district, if they are of various and different natures, than if they are
all alike. There are trees (for example, the oak) on which a couple of
hundred of different species of insects live together. Some feed on the
fruits of the tree, others on the leaves, others again on the bark, the
root, etc. It would be quite impossible for an equal number of
individuals to live on this tree if all were of one species; if, for
example, all fed on the bark, or only upon the leaves. Exactly the same
is the case in human society. In one and the same small town, only a
certain number of workmen can exist, even when they follow different
occupations. The division of labour, which is of the greatest use to the
whole community, as well as to the individual workman, is a direct
consequence of the struggle for life, of natural selection; for this
struggle can be sustained more easily the more the activities, and
hence, also, the forms of the different individuals deviate from one
another. The different function naturally produces its reaction in
changing the form, and the physiological division of labour necessarily
determines the morphological differentiation, that is, the “divergence
of character.”(37)
Now, I beg the reader again to remember that all species of animals and
plants are variable, and possess the capability of adapting themselves
to different places or to local relations. The varieties or races of
each species, according to the laws of adaptation, deviate all the more
from the original primary species, the greater the difference of the new
conditions to which they adapt themselves. If we imagine these
varieties—which have proceeded from a common primary form—to be
disposed in the shape of a branching, radiating bunch, then those
varieties will be best able to exist side by side and propagate which
are most distant from one another, which stand at the ends of the
series, or at the opposite sides of the bunch. Those forms, on the other
hand, occupying a middle position—presenting a state of
transition—have the most difficult position in the struggle for life.
The necessaries of life differ most in the two extremes, in the
varieties most distant from one another, and consequently these will get
into the least serious conflict with one another in the general struggle
for life. But the intermediate forms, which have deviated less from the
original primary form, require nearly the same necessaries of life as
the original form, and therefore, in competing for them, they will have
to struggle most with, and be most seriously threatened by, its members.
Consequently, when numerous varieties of a species live side by side on
the same spot of the earth, the extremes, or those forms deviating most
from one another, can much more easily continue to exist beside one
another than the intermediate forms which have to struggle with each of
the different extremes. The intermediate forms will not be able to
resist, for any length of time, the hostile influences which the extreme
forms victoriously overcome. These alone maintain and propagate
themselves, and at length cease to be any longer connected with the
original primary species through intermediate forms of transition. Thus
arise “good species” out of varieties. Thus, then, the struggle for life
necessarily favours the general divergence of organic forms, that is,
the constant tendency of organisms to form new species. This fact does
not rest upon any mystic quality, or upon an unknown formative tendency,
but upon the interaction of Inheritance and Adaptation in the struggle
for life. As the intermediate forms, that is, the individuals in a state
of transition, of the varieties of every species die out and become
extinct, the process of divergence constantly goes further, and from the
extremes forms develop which we distinguish as new species.
Although all naturalists have been obliged to acknowledge the
variability and mutability of all species of animals and plants, yet
most of them have hitherto denied that the modification or
transformation of the organic form surpasses the original limit of the
characters of the species. Our opponents cling to the
proposition—“However far a species may exhibit deviations from its
usual form in a collection of varieties, yet the varieties of it are
never so distinct from one another as two really good species.” This
assertion, which Darwin’s opponents usually place at the head of their
arguments, is utterly untenable and unfounded. This will become quite
clear as soon as we critically compare the various attempts to define
the idea of species. No naturalist can answer the question as to what is
in reality a “genuine or good species” (“bona species”); yet every
systematic naturalist uses this expression every day, and whole
libraries have been written on the question as to whether this or that
observed form is a species or a variety, whether it is a really good or
a bad species. The most general answer to this question used to be the
following: “To one species belong all those individuals which agree in
all essential characteristics. Essential characteristics of species are
those which remain permanent or constant, and never become modified or
vary.” But as soon as a case occurred in which the characteristic—which
had hitherto been considered essential—did become modified, then it was
said, “This characteristic is not essential to the species, for
essential characteristics never vary.” Those who argued thus evidently
moved in a circle, and the naïveté with which this circular method of
defining species is laid down in thousands of books as an unassailable
truth, and is still constantly repeated, is truly astonishing.
All other attempts which have been made to arrive at a definite and
logical determination of the idea of organic “species” have, like the
last, been utterly futile, and led to no results. Considering the nature
of the case, it cannot be otherwise. The idea of species is just as
truly a relative one and not absolute, as is the idea of variety, genus,
family, order, class, etc. I have proved this in detail in the criticism
of the idea of species in my “General Morphology” (Gen. Morph. ii.
323-364). I will waste no more time on this unsatisfactory discussion,
and now only add a few words about the _relation of species to
hybridism_. Formerly it was regarded as a dogma, that two good species
could never produce hybrids which could reproduce themselves as such.
Those who thus dogmatized almost always appealed to the hybrids of a
horse and donkey, the mule and the hinny, which, truly enough, are
seldom able to reproduce themselves. But the truth is that such
unfruitful hybrids are rare examples, and in the majority of cases
hybrids of two totally different species are fruitful and able to
reproduce themselves. They can almost always fruitfully mix with one or
other of the parent species, and sometimes also among themselves; and in
this way completely new forms can originate according to the laws of
“mixed transmission by inheritance.”
Thus, in fact, _hybridism is a source of the origin of new species_,
distinct from the source we have hitherto considered—natural selection.
I have already spoken occasionally of these _hybrid species_ (species
hybridæ), especially of the hare-rabbit (Lepus Darwinii), which has
arisen from the crossing of a male hare and a female rabbit; the
goat-sheep (Capra ovina), which has arisen from the pairing of a he-goat
and ewe; also the different species of thistles (Cirsium), brambles
(Rubus), etc. It is possible that many wild species have originated in
this way, as even Linnæus assumed. At all events, these hybrid species,
which can maintain and propagate themselves as well as pure species,
prove that hybridism cannot serve in any way to give an absolute
definition to the idea of species.
I have already mentioned (p. 47) that the many vain attempts to define
the idea of species theoretically have nothing whatever to do with the
practical distinction of species. The extensive practical application of
the idea of species, as it is carried out in systematic zoology and
botany, is very instructive as furnishing an example of human folly.
Hitherto, by far the majority of zoologists and botanists, in
distinguishing and describing the different forms of animals and plants,
have endeavoured, above all things, to distinguish accurately kindred
forms as so many “good species.” However, it has been found scarcely
possible, in any group, to make an accurate and consistent distinction
of such “genuine or good species.” There are no two zoologists, no two
botanists, who agree in all cases as to which of the nearly related
forms of a genus are good species, and which are not. All authors have
different views about them. In the genus _Hieracium_, for example, one
of the commonest genera of European plants, no less than 300 species
have been distinguished in Germany alone. The botanist Fries, however,
only admits 106, Koch only 52, as “good species,” and others accept
scarcely 20. The differences in the species of brambles (Rubus) are
equally great. Where one botanist makes more than a hundred species, a
second admits only about one half of that number, a third only five or
six, or even fewer species. The birds of Germany have long been very
accurately known. Bechstein, in his careful “Natural History of German
Birds,” has distinguished 367 species, L. Reichenbach 379, Meyer and
Wolff 406, and Brehm, a clergyman learned in ornithology, distinguishes
even more than 900 different species.
Thus we see that here, and, in fact, in every other domain of systematic
zoology and botany, the most arbitrary proceedings prevail, and, from
the nature of the case, must prevail. For it is quite impossible
accurately to distinguish varieties and races from so-called “good
species.” _Varieties are commencing species._ The variability or
adaptability of species, under the influence of the struggle for life,
necessitates the continual and progressive separation or differentiation
of varieties, and the perpetual delimitation of new forms. Whenever
these are maintained throughout a number of generations by inheritance,
whilst the intermediate forms die out, they form independent “new
species.” The origin of new species by division of labour, or
separation, divergence, or differentiation of varieties, is therefore a
_necessary consequence of natural selection_.(37)
The same kind of interest attaches to a second great law which we deduce
from natural selection, and which is, indeed, closely connected with the
law of Divergence, but in no way identical with it; namely, the law of
_Progress_ (progressus), or _Perfecting_ (teleosis). (Gen. Morph. ii.
257.) This great and important law, like the law of differentiation, had
long been empirically established by palæontological experience, before
Darwin’s Theory of Selection gave us the key to the explanation of its
cause. The most distinguished palæontologists have pointed out the law
of progress as the most general result of their investigations of fossil
organisms. This has been specially done by Bronn, whose investigations
on the laws of construction(18) and the laws of the development(19) of
organisms, although little heeded, are excellent, and deserve most
careful consideration. The general results of the law of differentiation
and the law of progress, at which Bronn arrived by a purely mechanical
hypothesis, and by exceedingly accurate, laborious, and careful
investigations, are brilliant confirmations of the truth of these two
great laws which we deduce as necessary inferences from the theory of
selection.
The law of progress or of perfecting establishes the exceedingly
important fact, on the ground of palæontological experience, that in
successive periods of this earth’s history, a continual increase in the
perfection of organic formations has taken place. Since that
inconceivably remote period in which life on our planet began with the
spontaneous generation of Monera, organisms of all groups, both
collectively as well as individually, have continually become more
perfectly and highly developed. The steadily increasing variety of
living forms has always been accompanied by progress in organization.
The lower the strata of the earth in which the remains of extinct
animals and plants lie buried, that is, the older the strata are, the
more simple and imperfect are the forms which they contain. This applies
to organisms collectively, as well as to every single large or small
group of them, setting aside, of course, those exceptions which are due
to the process of degeneration, which we shall discuss hereafter.
As a confirmation of this law I shall mention only the most important of
all animal groups, the tribe of vertebrate animals. The oldest fossil
remains of vertebrate animals known to us belong to the lowest class,
that of Fishes. Upon these there followed later more perfect Amphibious
animals, then Reptiles, and lastly, at a much later period, the most
highly organized classes of vertebrate animals, Birds and Mammals. Of
the latter only the lowest and most imperfect forms, without placenta,
appeared at first, such as are the pouched animals (Marsupials), and
afterwards, at a much later period, the more perfect mammals, with
placenta. Of these, also, at first only the lower kinds appeared, the
higher forms later; and not until the late tertiary period did man
gradually develop out of these last.
If we follow the historical development of the vegetable kingdom we
shall find the same law operative there. Of plants there existed at
first only the lowest and most imperfect classes, the Algæ or tangles.
Later there followed the group of Ferns or Filicinæ (ferns, pole-reeds,
scale-plants, etc.). But as yet there existed no flowering plants, or
Phanerogama. These originated later with the Gymnosperms (firs and
cycads), whose whole structure stands far below that of the other
flowering plants (Angiosperms), and forms the transition from the group
of fern-like plants to the Angiosperms. These latter developed at a
still later date, and among them there were at first only flowering
plants without corolla (Monocotyledons and Monochlamyds); only later
were there flowering plants with a corolla (Dichlamyds). Finally, again,
among these the lower polypetalous plants preceded the higher
gamopetalous plants. The whole series thus constitutes an irrefutable
proof of the great law of progressive development.
Now, if we ask what is the cause of this fact, we again, just as in the
case of differentiation, come back to natural selection in the struggle
for life. If once more we consider the whole process of natural
selection, how it operates through the complicated interaction of the
different laws of Inheritance and Adaptation, we shall recognize not
only divergence of character, but also the perfecting of structure to be
the direct and necessary result of it. We can trace the same thing in
the history of the human race. Here, too, it is natural and necessary
that the progressive division of labour constantly furthers mankind, and
urges every individual branch of human activity into new discoveries and
improvements. This progress itself universally depends on
differentiation, and is consequently, like it, a direct result of
natural selection in the struggle for life.