If two crows be taken to an ornithologist and he be told that one of them
was caught in the Himalayas while the other was captured in Madras, he
will not be able to tell which individual came from which area: in other
words, the crows of Madras resemble those of the Himalayas. This, of
course, is no unusual phenomenon. The same may be said of the myna, the
king-crow, and a great many other birds and beasts. Yet the phenomenon is
a remarkable one if we take into account the facts of variation.
If several hundred thousand crows be collected and carefully examined, it
will be found that no two of them resemble one another in all respects.
This being so, we should expect the crows of Madras to differ from those
of the Himalayas, since the two environments are so dissimilar. We may
say with tolerable certainty that no intercrossing takes place between
the crows of the two localities: for these birds are stay-at-home
creatures, and do not wander far afield. In this case, therefore, it is
not intercrossing that has prevented the origin of local races.
A consideration of the main causes which conduce to the stability of
species may not be devoid of interest; for the subject is one which has
hitherto attracted but little attention. Since the Darwinian hypothesis
was given to the world we have heard so much of variation and the origin
of new species that the other phenomenon—that of the fixity of species—in
spite of varying environments has been almost entirely overlooked. Yet it
was just this feature of animal life that attracted the attention of the
older zoologists and led them to believe that species had been created
once and for all, and that, when created, they were immutably fixed.
Most biologists, if asked to explain the comparative fixity of species,
the slowness of evolution, would, I think, refer to the fact that
variations appear to take place indiscriminately in all directions. Take,
for example, a large number of birds of any species and measure any one
organ, let us say the first primary wing feather. Suppose the average
length be six inches. We shall find that in a considerable percentage of
the individuals measured the wing is exactly six inches in length: that
six inches is what we may call the favourite or fashionable length of the
wing. The next commonest lengths will be 5.99 and 6.01 inches, and so on.
We shall find that only a very small percentage of the individuals have
wings shorter than 5½ inches or longer than 6½ inches; and if we measured
a thousand individuals we probably should not find any in which the wing
was shorter than five inches or longer than seven.
Now, the commonly accepted theory is that in those cases where there is
free interbreeding the long-winged varieties and the short-winged
varieties tend to neutralise one another, hence no change in character
takes place. The effects of variation are swamped by intercrossing. It is
only when intercrossing is checked, as when natural selection weeds out
certain varieties, that evolution occurs.
This theory, of course, explains, or helps to explain, why species are so
stable; but it involves the assumption that there is no such thing as
sexual selection among animals in a state of nature. The theory assumes
that individuals mate in a haphazard manner, that a long-winged hen is as
likely to select a short-winged husband as a long-winged one. Are we
justified in assuming this? At present there is little evidence on the
subject. Evidence can only be procured by measuring a number of pairs of
birds that have mated, and seeing whether large hens mate chiefly with
large cocks or with small cocks, or indifferently with large or small
cock-birds.
That sexual selection is a reality and not a mere hypothesis there can, I
think, be but little doubt. It is with the theory that supposes that the
females alone exercise selection that I feel compelled to quarrel. The
male selects his partner just as much as the female selects hers. The
choice is mutual.
In the Zoological Gardens at Lahore there are a number of ordinary
coloured peacocks and a number of albinos. No coloured hen will mate with
a coloured cock if she is allowed to exercise a choice between him and an
albino. Here, then, is a clear example of sexual selection.
Professor Karl Pearson has spent much time in trying to discover whether
there is such a thing as sexual selection—what we may call unconscious
selection—among human beings. His experiments tend to show that there is.
If we take a thousand married men whose stature is not less than six
feet, and a thousand also who are none of them taller than 5 ft. 8 in.,
we shall find that the average height of the wives of the former is
greater than that of the wives of the shorter men.
If wild animals display a similar characteristic, it is evident that to
say that intercrossing swamps variation and causes species to remain
stable is not altogether accurate; for, if like select like as partners,
we should expect a number of races to rapidly arise, or, at any rate,
three races—a large, medium, and small one. So far, however, as we can
see, species display no such tendency. We are therefore driven to the
conclusion either that there is among species in a state of nature no
tendency for like individuals to select like as their partners, or, if
there be such a tendency, there is some force at work which counteracts
it.
It may be thought that the case of the peafowl in the Lahore “Zoo” tends
to show that among animals it is dissimilarity, not similarity, that
attracts, for the coloured hens mate with white cocks in preference to
those like themselves.
As a matter of fact the hens select the white cocks, not because they are
white, but because of the strength of the sexual instincts of these
latter. The white cocks continually show off before the hens; the sexual
desire is developed more highly in them than in the ordinary cocks, and
it is this that attracts the hens.
We must also bear in mind that abnormal variations have a strong tendency
to perpetuate themselves. If a white cock mates with an ordinary peahen,
the majority of the offspring are pure white.
If there be such a thing as sexual selection, and if it be, as I believe,
the strongest, the most mettlesome individuals, those in which the sexual
instincts reach the highest development, that attract the opposite sex,
then the question arises: is there any connection between these
characteristics and the size and colour of their possessor? We are not in
possession of sufficient data to answer this question in the affirmative.
Nevertheless I believe that such a relation does exist.
The researches of Professor Pearson seem to point to the fact that there
exists a definite relation between variation and fertility. For every
species there is a mode or typical size and form, and from this there are
deviations in all directions, and, speaking generally, the greater the
deviation from the mode the less the fertility of the individual.
If this be a general law we have here a very potent factor tending to
make species stable. Those individuals which deviate least from the
common type are the most fertile; they produce the most offspring;
moreover, they are the most numerous, hence they, by sheer force of
numbers, keep a species stable. The abnormal individuals are
comparatively few in number, and they beget comparatively few of their
kind, so have no chance of establishing themselves and crushing out the
normal type, unless natural selection steps in to their aid.
Is comparative infertility the result of feebleness of the sexual
instinct? If so, sexual selection must be conducive to the stability of
species.
For if the rule be the greater the deviation of an individual from the
normal the less the development in it of the sexual instinct and the less
its fertility, it follows that an abnormal organism is less likely to
find a mate than a normal individual is; and if it do succeed in forming
a union, that union will probably produce less than the average number of
offspring.