Natural Selection and Evolutionary Progress

Abstract

RATE-GENES AND SELECTION THE results of selection at one period of the life-cycle may have repercussions on other periods and affect the species as a whole in unexpected ways. Perhaps the best example is that of intra-uterine selection in polytocous mammals, where rapidity of growth must be at a premium. This is likely to be transferred in whole or in part to post-natal life; intra-uterine selection may thus help to account for the progressive increase in size seen in so many mammalian lines during their evolution. At any rate, the converse seems to hold. One of the most characteristic features of man is a slowing down of general rate of development. Without it he could not in all probability have become fully human or biologically dominant. This condition could not have occurred until after man's ancestors ceased to have litters and began to bring forth a single young at a birth.