heterochrony are acceleration and retardation. Our paedomorphic features are linked to retardation in development; it is to this retardation that we must fix our attention.

Retardation in Human Evolution

Although Bolk's theory has been rejected, his central insight may be reinstated as the foundation of a modern analysis: retardation in development must be distinguished from fetalization of form—the first is a process that may or may not yield the second as a result. Curiously, this point has generally been lost in the enumerative tradition that followed Bolk's pronouncements. The evidences of retarded development have simply been included within lists of paedomorphic characters, and Bolk's distinction has been blurred.

I believe that human beings are "essentially" neotenous, not because I can enumerate a list of important paedomorphic features, but because a general, temporal retardation of development has clearly characterized human evolution. This retardation established a matrix within which all trends in the evolution of human morphology must be assessed. This matrix does not in itself guarantee a central role for paedomorphosis,⁶ but it certainly provides a mechanism for such a result, if this result be of selective value. This mechanism was utilized again and again in human evolution because retarded development carried a set of potential consequences with it: prolongation of fetal growth rates leading to larger sizes* and the retention of juvenile proportions. Is not such a system the proximate cause for evolutionary increase of the human brain?

Slijper (1936) based a famous critique of human neoteny upon the

* Or, if the feature is one that develops late in ontogeny, the retention of fetal growth tendencies usually leads to a reduction in size and the retention of juvenile proportions; an example is the human face. Several authors have fallen into the trap of requiring a correspondence in size and internal arrangement between juvenile ancestor and adult descendant before making an assessment of paedomorphosis. Thus, since the human brain has added so many billions of neurons to the feature of similar shape in ancestral juveniles, paedomorphosis is denied. "In the evolution of man from his ancestors, neoteny has taken place in respect of several features which show fetalization. At the same time, of course, in other directions, the evolution of man has involved progressive change of vast importance, some of which, however, might not have been possible (e.g. the development of the brain) had it not been for certain features of neoteny (e.g. the delay in closing the sutures of the skull)" (De Beer, 1958, p. 76). But this represents another pitfall in the use of static morphology as a criterion; rates and processes of growth are the appropriate standard—and the human brain is paedomorphic because it has increased by prolonging to later times and larger body size (even past birth) the characteristic positive allometry generally confined to fetal stages in primates and other mammals.