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.