Essays of a Biologist

by Julian Huxley

As a second result, we find that in vertebrates the gonads form part of what has been called the chemical directorate of the body—the interlocking system of endocrine glands, each of which is exerting an effect upon the rest. The importance of this is seen in the experiments of Steinach, Sand, Voronoff, and others, who have been able to obtain a rejuvenating effect in senile mammals by increasing, by various methods, the amount of secreting reproductive organ in the body.[32]

To what then has our rapid survey led us? The actual origin of sex is lost to us in the mists of a time inconceivably remote. Its preservation once in existence, and its present all-but-universal distribution seem to be definitely associated with the biological advantage of the plasticity which it confers. Later, the primary difference between male and female—their power of producing different sorts of reproductive cells—leads on to secondary differences. These differences may be biologically speaking non-significant, mere accidents of the primary difference. Or they may be in the nature of a division of labour between the sexes, this division of labour usually concerning the protection of the embryo or the care of the young, or more rarely the preservation of the individual itself. Or, finally, they may concern the more efficient union of the gametes; such differences may merely affect the ducts and apertures of the reproductive system, and be more or less mechanical; or they may concern the use of these systems, in the form of still mechanical instincts, or they may be concerned in some way or other with the emotional side of the animals, and consist in characters and actions which stimulate the emotions of the other sex, characters which we have termed epigamic.

It is only in higher groups that these emotion-stimulating sexual characters arise, for only in them has mind reached a sufficient degree of perfection. But even though detailed study reveals in a bird or a mammal a mental life of a complexity far more considerable than the average man would imagine, yet on the whole it is straightforward and its currents run fairly direct from stimulus to fulfilment.

When we reach man, however, the whole aspect of the matter changes. The change is most marked, naturally, in his mental organization. Through his powers of rapid and unlimited association, any one part of his experience can be combined with any other; through his powers of generalizing and of giving names to things, his experience is far more highly organized than that of any animal; through speech and writing he is inheritor of a continuous tradition which enormously enlarges his range of experience. Again, he can frame a purpose and thus put the objective of his actions far further into the future than can lower organisms.

There are, however, also changes of considerable biological importance on the physical side. Man brings with him from his animal ancestors the endocrine secretory mechanism of the reproductive organs: but his life is not subordinated to it in such an iron-bound way. To start with he has gradually lost all semblance of a breeding-season. Traces of it survive in some primitive races, but in civilized communities all one can say is that the number of births may show a slight seasonal variation; and the reproductive organs are capable of function in all twelve months of the year—a state of affairs known, I believe, in no other vertebrate, or at least in no wild species.[33]

In the second place, there has been in the female a further emancipation of the sexual life. In all other mammals the female will only receive the male at certain well-defined periods, which in their turn depend on cyclical changes in the ovaries. In man this restriction has been overcome, and, in spite of the survival of a certain degree of cyclical change in feeling, neither sex is restricted any longer to certain physically-determined periods for the consummation of its sexual life. This is, we may say, a triumph of mind over matter in the human organism, of the mental elements of the sexual life over the purely physical elements.

This is not to deny that the sexual life of man is dependent upon the reproductive hormones. It is apparently necessary for proper activation of the sexual centres in the brain that there should occur a continuous liberation of secretion from the reproductive organs into the blood. Again, the mental activities of man are so much more important than those of other forms that even the cessation of activity of the reproductive organs, for instance in the female at the change of life, or even their total removal, need not prevent the continuation, albeit in a modified form, of the sexual life in its varied indirect manifestations.

Before attempting to probe the intricacies of the mental side of the subject, we had better see what we can learn of the physical. Let us first remind ourselves of one or two facts gained from animal experimentation. In the first place, in mammals the activation of the sexual instincts of one or the other sex appears to be completely or almost completely under the control of the internal secretions of the reproductive organs. Steinach and others have taken new-born male guinea-pigs and have removed their testes and grafted ovaries in their place. The result has been an animal almost completely feminized both as regards body and mind. In some of the animals milk was secreted, and when this occurred they would act as foster-mothers to new-born guinea-pigs of other parents. The reverse operation, the masculinization of females, was equally successful, the animals growing large and showing all the instincts of a normal male and none of those of a normal female.

A similar dependence of behaviour on gonad is seen in fowls. Here nature makes a number of experiments, which have recently been studied by Dr. Crew of Edinburgh. When the ovaries of a hen are affected by a certain type of tumour, the bird stops laying, her comb and wattles enlarge to the size of a cock’s, her spurs grow, she begins to crow, her plumage changes at the moult and becomes cock-like, and finally she becomes indistinguishable from a male. Indistinguishable, even in behaviour: her years of feminine routine in laying and brooding are forgotten: the secretion of the altered ovary now apparently resembles that of a testis and stimulates centres of the brain which would otherwise have remained permanently dormant. She struts and crows, fights and mates, and the memory of the previous part of her life is for all practical purposes lost, since the centres for female activity are no longer stimulated at all.

Various workers have even experimentally produced a state of hermaphroditism in mammals by simultaneous grafting of portions of testes and ovary: the behaviour here oscillates between male and female.[34]