XI - The Croonian Lecture. Sexual periodicity and the causes which determine it

Abstract

It is a matter of common knowledge that the great majority of animals, both vertebrate and invertebrate, not to mention plants, have a more or less definite season or seasons of the year at which they breed. This time for breeding is generally, though by no means invariably, in the spring and summer, and it is well known that whereas a favourable season as regards warmth and general conditions tends to accelerate breeding an unfavourable one may retard it. So much is known to be generally true, yet the precise factors which determine the sexual season vary in passing from group to group and from species to species or even from breed to breed. Westermarck (1921), confining himself to mammals alone, has pointed out that there is no month of the year at which some species does not have its breeding season, and yet that for the particular species in question the season is most regular. Speaking teleologically, the breeding season is regulated by the times most suitable for the young to be produced and reared. Without disparaging the use of teleological categories which justify themselves as means of generalization and prediction and are very generally used by the naturalist to the great advantage of his work, it is obvious that such a view is no explanation of the physiological causes of sexual periodicity in the individuals of which a species is made up. We still know only a little about these causes. But in view of the general correlation between the seasonal and the sexual cycles it must be assumed that these stand in the relation of cause to effect, unless, indeed, we believe in a pre-established harmony. And nowadays it is not fashionable to believe in pre-established harmonies. Moreover, in countries where conditions are more or less uniform throughout the year, as in some parts of the tropics, e.g., in the Philippines as found by Semper (1881), animals of all kinds may breed at any time.* This is not saying that there is no internal rhythm occurring independently of the environment.