is small, there must be great fertility to compensate for the consequent mortality; otherwise the race must die out. When, on the contrary, high endowments give much capacity for self-preservation, a correspondingly low degree of fertility is requisite," lest the rate of multiplication should outrun the supply of food.[1] In general, then, there is an opposition of individuation and genesis, or individual development and fertility. The rule holds for groups and species more regularly than for individuals: the more highly developed the species or the group, the lower will its birth-rate be. But it holds for individuals too, on the average. For example, intellectual development seems hostile to fertility. "Where exceptional fertility exists, there is sluggishness of mind, and where there has been, during education, excessive expenditure in mental action, there frequently follows a complete or partial infertility. Hence the particular kind of further evolution which Man is hereafter to undergo is one which, more than any other, may be expected to cause a decline in his power of reproduction."[2] Philosophers are notorious for shirking parentage. In woman, on the other hand, the arrival of motherhood normally brings a diminution of intellectual activity;[3] and perhaps her shorter adolescence is due to her earlier sacrifice to reproduction.
Despite this approximate adaptation of birth-rate to the needs of group survival, the adaptation is never complete, and Malthus was right in his general principle that population tends to outrun the means of subsistence. "From the beginning this pressure of population has been the proximate cause of progress. It produced the original diffusion of the race. It compelled men to abandon predatory habits and take to agriculture. It led to the clearing of the earth's surface. It forced men into the social state,…and developed the social sentiments. It has stimulated to progressive improve-
