The Zoologist/4th series, vol 5 (1901)/Issue 725/Note on the Origin of Sexual Dimorphism, and of Nuptial Weapons and Ornamentation, Barrett-Hamilton

by Gerald Edwin Hamilton Barrett-Hamilton

 

NOTE ON THE ORIGIN OF SEXUAL DIMORPHISM,
AND OF NUPTIAL WEAPONS AND ORNAMENT-
ATION.

By G.E.H. Barrett-Hamilton.

 

Not long ago^[1] I suggested, from a consideration of the spawning habits of the various species of Oncorhynchus, that secondary sexual characters, as well as the seasonal assumption of nuptial ornaments and weapons, might have had their origin in pathological conditions: that the whole metabolism of the animal is upset in the effort to produce the sexual products, with a result that pigment and matter are set loose in the body, and find their way to new regions, often with a fatal—always with a serious—influence on the animal, but resulting in the production of nuptial adornments and weapons. Mr. Cunningham, in parts of his argument,^[2] comes very near this hypothesis—for instance, when he lays great stress on the influence of the veins as a controlling factor in the evolution of sexual characters. Nevertheless, he is most unfortunate to have missed the Reports of Investigations on the Life-History of Salmon, published by the Fishery Board of Scotland. In these papers both his and my views are supported, and it is clearly shown that in the breeding Salmon, whether male or female, such transference of pigment and matter, both of fats, proteids, phosphorus compounds, and iron, actually does take place.

There remains yet another point upon which I wish to touch. Many naturalists base their arguments on the absence of proof that acquired characters are or can be inherited. Here they have a strong position; but, considering the matter closely, do we find that secondary sexual characters are in all cases inherited? The form and detail of an animal's body exist as the expression of, or owe their structure to, two forces, the one purely vegetative, or due to purely nutritive causes, the other sexual.

Now, if we remove the generative organs—the origin of sexual activity—we leave an animal purely vegetative, and one in which none, or hardly any, of the secondary sexual characters will make their appearance. They are not then part of the essential vegetative basis of the animal, but a mere expression of its sex. Remove the sex, and we remove them also. Can it then be said that they are hereditary, even although the sexual activity from which they arise be so? I suggest that they are not, although their partial appearance in some cases, even after the destruction of the generative organs (if not due to an imperfect destruction), would seem to show that perhaps they may eventually, after many generations, become so.

What I conceive to happen is somewhat as follows:—In animals which exhibit neither sexual dimorphism nor seasonal armature or ornamentation, the influence of the generative organs is exerted equally upon the sexes, as well as, probably, throughout life. In animals exhibiting the phenomenon either of sexual dimorphism or seasonal armature or ornamentation, the generative organs, when the individual is young, have usually little or no influence on the body, which follows in its growth the simplest possible laws. As soon, however, as the generative organs commence to grow, their influence is usually very marked. Their increase—often sudden, and, one might almost say, violent—is effected at the expense of the other organs, which, as in the case of the muscles of the Salmon, are actually robbed of their material. The whole metabolism of the body is disturbed, and the nervous system is particularly affected. The pigment and material thus set loose is not necessarily transferred in its entirety to the genitalia, but may, as in the case of Oncorhynchus, find its way to the skin or elsewhere. I have suggested that in some such cases the condition of the animal is purely pathological. The heightened coloration is, as in the human jaundice, the mere outward manifestation of disease—a disease to which, in this case, the animal eventually succumbs. There must, however, be numerous cases where the animal, although sickening, survives. It is here that the power of Natural or Sexual Selection supervenes as a guiding influence on the manner and direction of the transference of pigment and matter. This matter, at first transferred haphazard, is guided into channels which bring it to those parts of the body which are most in use in courtship, or chiefly subjected to nuptial energy. Hence may result many nuptial weapons or ornaments. Under such an argument we at once understand how it is that in some animals the sexual characters are permanent, in others transitory. Inasmuch as they follow the growth of the genital organs, where this growth is periodical so are they periodical, and, where the genital organs are influential throughout life, the characters are permanent, waxing and waning, however, like the Stag's horns, with their progress from youth through maturity to senescence. What is inherited, then, may well be not the secondary sexual characters themselves, but the influence of the genitalia, the tendency to the disruption of spare material and its deposition in particular regions, a process which certainly appears to become fixed after numerous generations.

Such is a view of sexual dimorphism and the seasonal assumption of nuptial weapons or ornaments, which I venture to put forward in all humility. Two advantages may be claimed for it—(1) it is based on a physiological standpoint, and starts on firmer and deeper ground than the older theories; (2) it includes in its scope not only persistent sexual dimorphism, but seasonal exhibitions of sexuality.

 

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1. Cf. 'Proc. Camb. Phil. Soc' vol. x. pt. v. pp. 279–285.
2. "Sexual Dimorphism in the Animal Kingdom."