We must not, however, infer that this enhancement of the procreative power is to be attributed directly to “the different positions of the sun with respect to the earth,”[169] or to the temperature of a certain season. The phenomenon does not immediately spring from this cause in the case of any other animal species. Neither can it be due to abundance of food. In the northern parts of Europe many more conceptions take place in the months of May and June, when the conditions of life are often rather hard, than in September, October, and November, when the supplies of food are comparatively plentiful. In the north-western provinces of Germany, as well as in Sweden, the latter months are characterized by a minimum of conceptions.[170] Among the Kaffirs, more children are conceived in November and December than in any other month, although, according to the Rev. H. T. Cousins, food is most abundant among them from March to September. And among the Bateke, the maximum of conceptions falls in December and January, although food is, as I am informed by Dr. Sims, most plentiful in the dry season, that is, from May to the end of August.
On the other hand, the periodical increase of conceptions cannot be explained by the opposite hypothesis, entertained by some physiologists, that the power of reproduction is increased by want and distress. Among the Western Australians and Californians,[171] for instance, the season of love is accompanied by a surplus of food, and in the land of the Bakongo, among whom Mr. Ingham believes most conceptions to take place in December and January, food is, according to him, most abundant precisely in these months and in February.
It seems, therefore, a reasonable presumption that the increase of the sexual instinct at the end of spring or in the beginning of summer, is a survival of an ancient pairing season, depending upon the same law that rules in the rest of the animal kingdom. Since spring is rather a time of want than a time of abundance for a frugivorous species, it is impossible to believe that our early ancestors, as long as they fed upon fruits, gave birth to their young at the beginning of that period. From the statements of Sir Richard Burton and Mr. A. R. Wallace, already quoted,[172] we know that the man-like apes breed early in the season when fruits begin to be plentiful. But when man began to feed on herbs, roots, and animal food, the conditions were changed. Spring is the season of the re-awakening of life, when there are plenty of vegetables and prey. Hence those children whose infancy fell in this period survived more frequently than those born at any other. Considering that the parents of at least a few of them must have had an innate tendency to the increase of the power of reproduction at the beginning of summer, and considering, further, that this tendency must have been transmitted to some of the offspring, like many other characteristics which occur periodically at certain seasons,[173] we can readily understand that gradually, through the influence of natural selection, a race would emerge whose pairing time would be exclusively or predominantly restricted to the season most favourable to its subsistence. To judge from the period when most children are born among existing peoples, the pairing season of our prehistoric ancestors occurred, indeed, somewhat earlier in the year than is the case with the majority of mammalian species. But we must remember that the infancy of man is unusually long; and, with regard to the time most favourable to the subsistence of children, we must take into consideration not only the first days of their existence, but the first period of their infancy in general. Besides food and warmth, several other factors affect the welfare of the offspring, and it is often difficult to find out all of them. We do not know the particular circumstances that make the badger breed at the end of February or the beginning of March,[174] and the reindeer of the Norwegian mountains as early as April;[175] but there can be no doubt that these breeding seasons are adapted to the requirements of the respective species.