Just how many offspring the average family must have to meet these requirements is not easy to calculate precisely, as the number varies with the death-rate, the proportion of celibates and barren marriages, the rate of general increase and other factors. I have consulted several statistical experts, but found none of them willing to make a definite estimate for the United States. I should say, roughly, that a stock cannot hold its own in numbers with an average of less than four children to a fertile marriage, and considering the large general rate of increase in this country, five would probably be nearer the mark. A family of three children or less, where the parents are of good descent and, physically and as regards income, capable of having more, must be reckoned a “race-suicide” family, not doing its share in keeping up the stock. It was formerly assumed that the impulse to propagation, in human as in animal types, was to be taken for granted, the only question being how far the economic conditions would allow this impulse to become effective. A closer study shows that the control of society begins further back, and can easily modify the development of the instincts themselves in such a way that they cease to impel natural increase. Gratification of the sexual impulses may be separated from reproduction, and it may well come to pass that the classes in which they have the fullest sway are the least prolific. The maternal instinct, though less apt to lapse into sensuality, is not much more certain in its operation. It may expend itself on one or two children, or even be directed to other objects. Modern conditions tend strongly to what is called birth-control, that is, to making the number of children a matter of intention, and not of mere physiology. This is in accord with the general increase of choice, and we may hope that it will work out well in the long run, but it calls for a new conscience and a new intelligence in this connection. The old process did not require that people should know anything about eugenics, or feel the duty of raising a good-sized family; that was left to unconscious forces; but now that they are coming to have no more children than they want, it is evident that, unless those who represent the better strains want the requisite number, such strains must decline. And as birth-control prevails most in the intelligent classes, the possibility of deterioration is manifest. Only eugenic ideals and conduct can save from depletion those stocks which share most fully in the currents of progress. The fact that intelligence saves on the death-rate and enables the type to be maintained by a smaller number of births is of some moment, but we must not imagine that any saving of this sort will enable families of two or three children to keep up a thriving stock.