Abstract

t is abundantly evident that rates of evolution vary. They vary greatly from group to group, and even among closely related lineages there may be strikingly different rates. Differences in rates of evolution, and not only divergent evolution at comparable rates, are among the reasons for the great diversity of organisms on the earth. Among the living primates there are, for instance, some rather unspecialized or primitive prosimians (i.e., little changed from Eocene progenitors), a larger number of divergently specialized prosimians, many monkeys of different degrees of progression and divergence, a few apes, and the unique species of man. Important as is the purely divergent evolution, it is also clear that differential rates are involved. At the extremes, the lineages of the more primitive living prosimians have evolved less rapidly as regards the whole of their structure and adaptive position than has the lineage of man.