This, however, is only the more obvious part of the truth. More subtly the social condition determines how any hereditary type develops and whether it has a sort of life that is favorable to propagation. The whole process of survival is, from one point of view, a matter of social psychology. Psychological influences direct the development of the instincts, guiding the selection of one sex by the other, and of both by the social group. The question just how a hereditary type must be related to the social system in order to survive cannot be answered in any simple way. It is not safe to say that the most successful types, in a social sense, have the best chance of survival; such types often tend to sterility. This may take place through the absorption of their energies in social activities at the expense of propagation; also through overfeeding or lack of incentive, leading to moral decay. Nor do the types that fail socially necessarily fail to propagate, since traits like lack of foresight, which diminish success, may increase the number of offspring. In order that a hereditary type may survive equally with others the individuals belonging to it must bring to maturity at least as many children, in proportion to their number, as those of other types. It is not sufficient that those having children should rear enough of them to replace the parents; they must also compensate for several sources of loss. A considerable proportion of persons, from lack of vitality or other reasons, do not marry, or, being married, have no children, or lose those they have by early death. And, beyond this, there must be enough surplus of children to give the type they represent its share in the general increase of population. The failure of a part of the individuals of good stock to leave children is not necessarily a fault: in some degree it is an elimination of the weak that is essential to the welfare of the stock, whose vigor is not the same in all. Many of the celibate or sterile are such because they lack normal vitality. I think we can all find in our own circle of acquaintance people of excellent descent who are healthy enough, perhaps, but seem to lack that surplus of life which would make us feel that they are born to be fathers or mothers. At any rate, others must do what, for no matter what reason, they fail to do.