The cause of the winter maximum of conceptions, especially considerable among the peoples of Northern Europe, is generally sought in social influences, as the quiet ensuing on the harvest time, the better food, and the amusements of Christmas.[176] But the people certainly recover before December from the labours of the field, and Christmas amusements, as Wargentin remarks, take place at the end of that month and far into January, without any particular influence upon the number of births in October being observable.[177] It has, further, been proved that the unequal distribution of marriages over the different months exercises hardly any influence upon the distribution of births.[178] Again, among the Hindus the December and January maximum of conceptions seems from the lascivious festivities of several Indian peoples to be due to an increase of the sexual instinct. According to Mr. Hill, this increase depends upon healthy conditions with an abundant food supply. But, as I have already said, it is not proved that a strengthened power of reproduction and abundance of food are connected with one another.
I am far from venturing to express any definite opinion as to the cause of these particular phenomena, but it is not impossible that they also are effects of natural selection, although of a comparatively recent date. Considering that the September maximum of births (or December maximum of conceptions) in Europe becomes larger the farther north we go; that the agricultural peoples of Northern Europe have plenty of food in autumn and during the first part of winter, but often suffer a certain degree of want in spring; and, finally, that the winter cold does not affect the health of infants, the woods giving sufficient material for fuel,—it has occurred to me that children born in September may have a better chance of surviving than others. Indeed, Dr. Beukemann states that the number of still-born births is largest in winter or at the beginning of spring, and that “the children born in autumn possess the greatest vitality and resisting power against the dangers of earliest infancy.”[179] This would perhaps be an adequate explanation either of an increase of the sexual instinct or of greater disposition to impregnation in December. It is not impossible either, that the increase of the power of reproduction among the Hindus in December and January, which causes an increase of births in September and October—_i.e._, the end of the hot season and the beginning of winter—owes its origin to the fact that during the winter the granaries get filled and some of the conditions of life become more healthy. But it should be remarked that September itself, according to Mr. Hill, is a very unhealthy month.[180]