Man, so the truism goes, lives increasingly in a man-made environment. This places a special burden on human immaturity, for it is plain that adapting to such variable conditions must depend very heavily on opportunities for learning, or whatever the processes are that are operative during immaturity. It must also mean that during immaturity man must master knowledge and skills that are either stored in the gene pool or learned by direct encounter, but which are contained in the culture pool--knowledge about values and history, skills as varied as an obligatory natural language or an optional mathematical one, as mute as levers or as articulate as myth telling. Yet, it would be a mistake to leap to the conclusion that because human immaturity makes possible high flexibility, therefore anything is possible for the species. Human traits were selected for their survival value over a four--to five-million-year period with a great acceleration of the selection process during the last half of that period. There were crucial, irreversible changes during that final man-making period: recession of formidable dentition, 50 percent increase in brain volume, the obstetrical paradox- bipedalism and strong pelvic girdle, larger brain through a smaller birth canal--immature brain at birth, and creation of what Washburn has called a 'technical-social way of life,' involving tool and symbol use. Note, however, that hominidization consisted principally of adaptations to conditions in the Pleistocene. These preadaptations, shaped in response to earlier habitat demands, are part of man's evolutionary inheritance. This is not to say that close beneath the skin of man is a naked ape, that civilization is only a veneer. The technical-social way of life is a deep feature of the species adaptation. But we would err if we assumed a priori that man's inheritance placed no constraint on his power to adapt. Some of the preadaptations can be shown to be presently maladaptive. Man's inordinate fondness for fats and sweets no longer serves his individual survival well. And the human obsession with sexuality is plainly not fitted for survival of the species now, however well it might have served to population the upper Pliocene and the Pleistocene. Nevertheless, note that the species responds typically to these challenges by technical innovation rather than by morphological or behavioral change. Contraception dissociates sexuality from reproduction. We do not, of course, know what kinds and what range of stresses are produced by successive rounds of such technical innovation. Dissociating sexuality and reproduction, for example, surely produces changes in the structure of the family, which in turn redefine the role of women, which in turn alters the authority pattern affecting the child, etc. continuing and possible acceleration change seems inherent in such adaptation. And this, of course, places and enormous pressure on man's uses of immaturity, preparing the young for unforeseeable change-the more so if there are severe restraints imposed by human preadaptations to earlier conditions of life. It can be inferred that the obstetrical paradox is puzzling because ______.