changes have been brought about, which are not only of no use to their
possessors, but would be actually disadvantageous if they were living
under natural conditions. Some of the very short-billed breeds of
pigeons have the bill so short and soft that the young can no longer
use it to scratch and break the egg-shell, and would perish miserably
if human aid were not at hand. The Yorkshire pig has become such a
colossus of fat on weak, short legs, that if it were dependent on
its own resources, it could not secure its food, much less escape
from a beast of prey; and among horses the heavy cart-horse and the
racer are alike unfit to cope with the dangers of a wild life, or the
vicissitudes of weather.
Breeding has done much to bring about variations useful to man. Thus we
have breeds of cattle which excel in flesh, or in milk, or as draught
animals, and sheep which excel in flesh or in wool, and to what a
height the perfecting of a useful quality can be brought is shown, in
regard to fineness of wool, by that finest breed of sheep, the merino,
which instead of the 5,500 hairs borne by the old German sheep on a
square inch, possesses 48,000.
Not infrequently it is a particular stage of a species that has
been bred by man, and the other stages have remained more or less
unaltered. Thus it is with one of the few domesticated insects, the
silk-moth. Only the cocoon is of use to man, and according to the
cocoon different breeds are distinguished, differing in fineness,
colour, &c.; but no breeds can be distinguished in reference to the
larvæ, or the perfect insects. Among gooseberries there are about a
hundred varieties distinguished according to the form, colour, size,
thickness of skin, hairiness, &c., of the fruits, but the little,
inconspicuous, green blossoms, of which the breeders take no account,
are alike in them all. In the pansies (_Viola tricolor_), on the other
hand, it is only by the flowers that the varieties are distinguished,
while the seeds have remained alike in all.
It may be asked how it could have occurred to any one, when pigeons,
for instance, first began to be domesticated, to wish to produce
fantails or pouters, since he could have no mental picture of them
in advance. Darwin replies to this objection, that it was not always
conscious and methodical artificial selection, such as is now
practised, that brought about the origin of breeds, but that they have
very often resulted, and at first perhaps always, from unconscious
selection. When savages tamed a dog, they used the 'best' of their dogs
for breeding, that is, they chose those which had in the highest degree
the qualities they valued, watchfulness, for instance, or if the dog
were intended for the chase, keen scent and swiftness. In this way the
body of the animal would be changed in a definite direction, especially
if rivalry helped, and if it was the ambition of each to possess a
dog as good as, or better than those of his tribal companions. That
perfectly definite changes in bodily form can thus be brought about
unconsciously is well illustrated by the case of a racehorse. This has
arisen within the last two hundred years simply because the fleetest
of the products of crossing between the Arab and the English horse
were always chosen for breeding. It could not have been predicted that
horses with thin neck, small head, long rump, and slender legs would
necessarily be the swiftest runners; but this is the form which has
resulted from the selection,--a very ugly, but very swift horse. This
unconscious selection must undoubtedly have played a large part in the
early stages of the evolution of the breeds of our domestic animals.
But even in the fully conscious and methodical selective breeding of
particular characters, the breeder rarely alters only the one his
attention is fixed on; generally quite a number of other characters
alter apart from his intention as an inevitable accompaniment of the
desired variation on which attention was riveted. There are breeds
of rabbits whose ears hang limply down instead of standing erect,
and in these so-called lop-eared rabbits the ear-muscles are partly
degenerated, and as a consequence of this lack of muscular strain the
skull has assumed another form. Thus the variation of one part may
influence the development of a second and a third organ, and may even
not stop there, for very often the influence has penetrated much deeper
and affected quite remote parts of the body.
If any one were to succeed in adding a heavy pair of horns to a breed
of hornless sheep, there would run parallel with the course of this
variation, which was directly aimed at, a long series of secondary
changes which would affect at least the whole of the anterior half of
the body; the skull would become thicker and stronger to support the
weight of the heavy horns; the neck-tendon (_ligamentum nuchæ_) would
have to become thicker to hold up the heavy head, and so also with the
muscles of the neck; the spinous processes of the cervical and dorsal
vertebrae would become longer and stronger, and the forelegs, too,
would need to adapt themselves to the heavier burden. Every organism
thus resembles, as it were, a mosaic, out of which no individual group
of pieces can be taken and replaced by another without in some measure
disturbing the correlation and harmony of the whole: in order to
restore this, the pieces all round about the changed part must be moved
or replaced by others.
According to Darwin, it is to this correlation of parts that we must
refer the variation of other parts besides the one intentionally
altered in the course of breeding. It must be admitted that the mutual
dependence of the parts plays a very important rôle in the economy
and development of the animal body, as we shall see later, and these
connexions still remain very mysterious to us. Especially is this
the case with the connexion between the reproductive organs and the
so-called secondary sexual characters. Removal of the reproductive
organs or gonads induces, in Man, for instance, if it be effected in
youth, the persistence of the childish voice and the non-development
of the beard; in the stag the antlers do not appear, and in the cock
the comb does not develop perfectly, &c., but we are not yet able to
understand clearly why this should be so.
LECTURE III
THE DARWINIAN THEORY (_continued_)
Natural selection--Variation--Struggle for existence--Geometric ratio
of rate of increase--Normal number and ratio of elimination in a
species--Accidental causes of extinction--Dependence of the strength
of a species on enemies--Struggle for existence between individuals
of the same species--Natural selection affects all organs and
stages--Summary.
IN artificial selection, through which, with or without conscious
intention, our domesticated animals and cultivated plants have arisen,
there must obviously be three kinds of co-operative factors: first, the
_variability_ of the species; second, the capacity of the organism for
_transmitting_ its particular characters to its progeny; and third, the
_breeder_ who selects particular qualities for breeding. No one of the
factors can be dispensed with; the breeder could effect nothing, were
there not presented to him the variations of parts in the particular
direction in which he wishes them to vary; an indefinite variation,
that is, a variation not guided by selection, would never lead to the
formation of new breeds; the species would probably become in time a
motley mixture of all sorts of variations, but a breed with definite
characters, transmissible in their purity to its descendants, could
never be formed. Finally, every process of selective breeding would be
futile, if the variations which appeared could not be transmitted.
Darwin assumes that processes of transformation quite similar to those
which take place under the guidance of Man occur also in nature,
and that it is mainly these which have brought about and guided the
transformations of species which have taken place in the course of the
earth's history. This process he calls _natural selection_.
It will readily be admitted that two out of the three factors necessary
to a process of selective breeding are present also in the natural
conditions of the life of species. Variability in some degree or
other is absent from no species of animal or plant, though it may be
greater in one than in another, and it cannot be doubted that the
inborn differences which distinguish one individual from another are
capable of transmission. It is only to untrained observers that all the
individuals of a species appear alike; for instance, all garden whites,
or all the individuals of the small tortoiseshell butterfly (_Vanessa
urticæ_), or all the chaffinches. If the individuals are carefully
compared it will be recognized that, even in these relatively constant
species, no individual exactly resembles another; that even among
butterflies twenty black scales may go to form a particular spot on
the wings in one individual and thirty or twenty-five in others; that
the length of the body, the legs, the antennæ, the proboscis exhibit
minute differences; and it is probable that the same combination of
quite similar parts never occurs twice. In many animals this cannot, of
course, be proved, because our power of diagnosis is not fine enough to
be able to estimate the differences directly, and because a comparison
of measurements of all the parts in detail is not practicable. So we
may here confine ourselves to the differences in the human race, which
we can recognize with ease and certainty. Even as regards the face
alone, all men differ from one another, and, numerous and complete as
likenesses may be, it is impossible to find two human beings in which
even the characters of the face are exactly similar. Even so-called
'identical twins' can always be distinguished if they are directly
compared either in person or in a photograph, and if the rest of the
body be also taken into consideration we find numerous small, sometimes
even measurable differences.
The same is true of animals, and it is only our lack of practice
that is at fault if we frequently fail to detect their individual
differences. The Bohemian shepherds are said to know personally, and be
able to distinguish from all the rest, every sheep in their herds of
many thousands. Thus the factors of variability and transmissibility
must be granted, and it remains only to ask: Who plays the part of
selecting breeder in wild nature? The answer to this question forms
the kernel to the whole Darwinian theory, which ascribes this rôle to
the conditions of life, to definite relations of individuals to the
external influences which they meet with during the course of their
lives, and which together make up their 'struggle for existence.'
To make this idea clear I must to some extent diverge.
It is a generally observed fact that, in every species of animals
or of plants, more germs and more individuals are produced than
grow to maturity, or become capable of reproduction. Numerous young
individuals perish at an early stage, often because of unfavourable
circumstances--cold, drought, damp, or through hunger, or at the hands
of their enemies. When we ask which of the progeny perish early, and
which survive to carry on the species, we are at first sight inclined
to suppose that this is entirely a matter of chance; but this is just
what Darwin disputed. It is not chance alone, it is, above all, the
differences between individuals, which enable them to withstand adverse
circumstances better or worse, and thus decide, according to his view,
which shall perish and which shall survive. If this be so, then we
have a veritable process of selection, and one which secures that the
'best,' that is, the most capable of resistance, survive to breed,
being thus, so to speak, 'selected.'
It may be asked, however, why so many individuals must perish in youth,
and whether it could not have been arranged that all, or at least most,
should survive till they had reproduced. But this is an impossibility,
unrealizable for this among other reasons, that organisms multiply in
geometrical progression, and that their progeny would very soon exceed
the limits of computability. This does not occur, for there is a limit
set which they can in no case overstep,--which, indeed, as we shall
see, they never reach--I mean the limits of space and food-supply.
Every species, by the natural requirements of its life, is restricted
to a particular habitat, to land or to water, but most are still more
strictly limited to a definite area of the earth's surface, which alone
affords the climate suited to them, or where alone the still more
specialized conditions of their existence can be realized. Thus, for
instance, the occurrence of a particular species of plant determines
that of the animal which is dependent on it for its food-supply. If
they could multiply unchecked, that is, without the loss of many of
their progeny, every species would fill up its area of occurrence and
exhaust the whole of its food-supply, and thus bring about its own
extermination. This seems to be prevented in some way, for as a matter
of fact it does not happen.
It may, perhaps, be imagined that this might be prevented by a
regulation of the productivity of the species, and that those which
have not a large area of distribution, or can only count on a
relatively limited food-supply, have also a low rate of multiplication,
but this is not the case; even the lowest rate of multiplication would
very soon suffice to make any species fill up its whole available space
and completely exhaust its food-supply. Darwin takes as an example
the elephant, which only begins to breed at thirty years of age, and
continues to do so till about ninety, but so slowly that in these
sixty years only three pairs of young are produced. Nevertheless, in
500 years an elephant pair would be represented by fifteen millions
of descendants, if all the young survived till they were capable of
reproduction. A species of bird with a duration of life of five years,
during which it breeds four times, producing and rearing four young
each time, would in the course of fifteen years have 2,000 millions of
descendants.
Thus, although the fertility of each species is, as a matter of fact,
precisely regulated, a low rate of multiplication is not in itself
sufficient to prevent the excessive increase of any species, nor is
the quantity of the relevant food-supply. Whether this be very large
or very small, we see that in reality it is never entirely used up,
that, as a matter of fact, a much greater quantity is always left over
than has been consumed. If increase depended only on food-supply, there
would, for instance, be food enough in their tropical home for many
thousand times more elephants than actually occur; and among ourselves
the cockchafers might appear in much greater numbers than they do even
in the worst cockchafer year, for all the leaves of all the trees are
never eaten up; a great many leaves and a great many trees are left
untouched even in the years when the voracious insects are the most
numerous. Nor do the rose-aphides, notwithstanding their enormously
rapid multiplication, ever destroy all the young shoots of a rose-bush,
or all the rose-bushes of a garden, or of the whole area in which roses
grow.
At the same time it must be noted, that the number of individuals in
a species undoubtedly does bear some relation to the amount of the
food-supply available; for instance, it is very low among the large
carnivores, the lion, the eagle, and the like. In our Alps the eagles
have become rarer with the decrease of game, and where one eagle pair
make their eyrie they rule alone over a hunting territory of more than
sixty miles, a preserve on which no others of the same species are
allowed to intrude. If there were several pairs of eagles in such a
preserve, they would soon have so decimated the food-supply that they
would starve. On the other hand, numerous herbivores, e.g. chamois and
marmots, live within the bounds of the pair of eagles' hunting grounds,
since the food they require is present in enormously greater quantity.
While it is true that the number of individuals of a given species
which live in a particular area is not exactly the same year in year
out, being subject to small, and sometimes, as in the case of the
aphides and cockchafers, to very great fluctuations, nevertheless we
may assume that the _average number_ remains the same, that in the
course of a century, or, let us say, of a thousand years, the number
of mature individuals inhabiting the particular area remains the
same. This, of course, only holds true on the supposition that there
has been no great change in the external conditions of life during
this period. But before Man began to interfere with nature, these
external conditions would remain uniform for much longer periods
than we have assumed. Let us call the average number of individuals
occurring on such a uniform area, _the normal number_ of the species;
this number will be determined in the first instance by the number of
offspring that are annually brought forth, and secondly by the number
that annually perish before reaching maturity. As the fertility of
a species is a definite quantity, so also will its elimination be
definite, or, as we may say, when the normal number under uniform
conditions of life remains constant, the ratio of elimination will
also remain constant. Each species is therefore subject to a perfectly
definite ratio of elimination which remains on the average constant,
and this is the reason why a species does not multiply beyond its
normal number notwithstanding the great excess of the food-supply, and
notwithstanding the fertility which, in all species, is sufficient to
lead to boundless multiplication.
It is not difficult to calculate the ratio of elimination for a
particular species, if one knows its rate of multiplication; for if
the normal number remains constant, it follows that only two of all
the offspring which a pair brings forth in the course of its life can
attain to reproductive maturity, and that all the rest must perish.
Suppose, for instance, a pair of storks produced four young ones
annually for twenty years, of these eighty young ones which are born
within this period, on an average seventy-eight must perish, and only
two can become mature animals. If more than two attained maturity
the total number of storks would increase, and this is against the
presupposition of constancy in the normal number. It is important, in
reference to the fact on which we are now focusing our attention, that
we should consider some other illustrations from the same point of
view. The female trout yearly produces about 600 eggs; let us assume
that it remains capable of reproduction for only ten years, then the
elimination-number of the species will be 6,000 less two, that is,
5,998, for of the 6,000 eggs only two can become mature animals. But
in the majority of fishes the ratio of extermination is enormously
greater than this. Thus a female herring brings forth 40,000 eggs
annually, the duration of life is estimated at ten years, and this
means an elimination number of 400,000 less two, that is, 399,998. The
carp produces 200,000 eggs a year, and the sturgeon two millions, and
both species live long, and remain capable of reproduction for at least
fifty years. But of all the 100 million eggs which are produced by the
sturgeon, only two reach their full development and reproduce; all
others perish prematurely.
But even with these examples we have not reached the highest
elimination number, for many of the lower animals--not to speak of
many plants--produce an even greater number of offspring. Leuwenhoek
calculated the fertility of a thread-worm at sixty million eggs, and a
tape-worm produces hardly less than 100 millions.
There exists, therefore, a constant relation between fertility and the
ratio of elimination; the higher the latter is, the greater must the
former be, if the species is to survive at all. The example of the
tape-worm makes this very obvious, for here we can readily understand
why the fertility must be so enormous, as we are aware of the long
chain of chances on which the successful development of this animal
depends. The common tape-worm of Man, _Tænia solium_, does not lay
its eggs, they remain enclosed within one of the liberated joints or
'proglottides.' Only if this liberated joint or one of the embryos
within it happens to be fortuitously eaten by a pig or other mammal can
there be successful development, and even then under difficulties and
possible failures, and not right away into adult animals, but first
into microscopically minute larvæ which may bore their way into the
walls of the intestine, or, if they are fortunate enough, may get into
the blood-stream and be carried by it to a remote part of the body.
There they develop into 'measles,' the so-called bladder-worms, within
which the head of the tape-worm arises. But in order that this may
become a complete and reproductive adult worm the pig must die, and the
next step necessary is that a piece of the flesh of the infected first
host must happen to be swallowed raw by a man or other mammal! Only
then does the fortunate bladder-worm--swallowed with the flesh--attain
the goal of its life, that is, a suitable place to mature in, the
food-canal of a human being. It is obvious that countless eggs must
be lost for one that succeeds in getting through the whole course of
a development depending so greatly on chance. Hence the necessity for
such enormous productivity of eggs.
In many cases the causes of elimination, which keep a species within
due bounds, are very difficult to determine. Enemies, that is to say,
other species which use the species in question as food, play an
important rôle; often, however, the cause lies in the unfavourableness
of external conditions, in chance, which is favourable only to one
of a thousand. The oak would only require to produce one seed in
the 500 years of its life, if it were certain that that one would
grow into an oak-tree; but most of the little acorns are eaten up by
pigs, squirrels, insects, &c., before they have had time to sprout,
thousands fall on ground already thickly covered with growth where
they cannot take root, and even if they do succeed in finding an
unoccupied space in which to germinate, the young plants are still
surrounded by a thousand dangers--the possibility of being devoured by
many animals large and small, of being suffocated by the surrounding
vegetation, and so on. We can thus understand, to some extent, though
only approximately, why it is that the oak must year by year produce
thousands of seeds in order that the species may maintain its normal
number, and not be exterminated; for it is obvious that a constant,
even though slow diminution of the normal number, a regular deficit, so
to speak, can end in nothing else than the gradual extinction of the
species.