to 1, 2 and 3. But we can not then account for the great number of zeros in the early decades, nor for the way in which the reduction of the variability occurs. Again it might be thought that there has been a growing reluctance to have families over a certain size, a reluctance that becomes more and more intense in the case of large sizes. But it is impossible to find any scale for the increase of this reluctance such that by assigning more and more individuals to the reluctant class we can derive a series of distributions by decades at all like those actually found.
Of course if we postulate both a lowering with time of the size to which families are restricted and a sliding scale of reluctance that also varies with time we can account for the observed facts. Such a hypothesis is, however, suspicious because of its complexity and apparent artificiality. I do not deny that it may be true, but until we find some further support for it, we are bound so far as the observed facts go to prefer the vera causa which explains the observations with perfect simplicity, and to attribute the numerical degeneration of our group to a real decrease in fertility.
So far as our general mental prepossessions go, however, a real decrease in fertility seems at first sight a preposterous doctrine. One can well imagine the sneer of the physician whose experience emphasizes the frequency of restriction and the pitying smile of the biologist who discerns that a progressive decrease in fertility of a species is a flat contradiction of the doctrine of natural selection. 'Play on with your statistical hair-splitting,' they would say, 'Nothing that you find will disturb our beliefs. We know better.'
But I venture to assert that the experiences of metropolitan physicians will not serve to prophecy the social psychology of the species we have studied, that their opinions may here be as wide of the mark as the common belief that unwillingness is the main cause of the failure of the women of the better classes to nurse their children. As to the contradiction of natural selection, I may suggest that the existence, amount and results of the elimination of types by their failure to produce their kind is after all a problem which only statistical inquiries can settle and that if the doctrine is to be used as an excuse for evading certain obvious facts in human history it is perhaps time that it should be questioned.
The issue is clear. The more fertile members of a race produce of course a larger measure of the next generation than do the less fertile. So also do their children, if fertility is inherited. There should then, according to present-day biology, be a quantitative evolution of fertility. Absolute sterility would needs be the first trait to be eliminated from a species. It should have disappeared from the human stock æons agro. And so long as there are variations in fertility and
