Birds of the Plains

by Douglas Dewar

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.