Rate of Evolution

Abstract

PROF. E. G. CONKLIN discusses (Scientific Monthly, 1920, vol. x., pp. 589–602) the difficult question of the rate of evolution, in-, eluding under evolution (a) diversification of species,(b) more perfect adaptation to the conditions of life, and (c) increasing differentiation and integration, or, more briefly, progress. If the rate of diversification (“divergent evolution”) depends upon the number of mutations that appear, Prof. Conklin argues that it should be proportional, other things being equal, to the rate of reproduction. But this does not seem to be the case. If the rate of improvement in adaptation (“adaptative evolution”) depends upon the rate of mutation and the severity of elimination, it also should be proportional to the rate of reproduction; but the finely adapted birds and mammals have a relatively low rate of reproduction. If the rate of “progressive evolutionl” depends upon the rate of mutation and the severity of selection, it again should be proportional to the rate of reproduction; but the most complex and most highly differentiated of all animals have the lowest rate of reproduction. In face of the difficulty of accounting for the differences in the rate of evolution, Prof. Conklin doubts whether current theories as to the causes of evolution are wholly satisfactory. It may be doubted, however, whether we are able to state the problems of diversity of rate with sufficient precision to allow of their being used as tests of the validity of the aetiological formulæ in the field. It is likely enough that there are factors of organic evolution still to be discovered, but we do not think that Prof. Conklin exhausts the potency of those that are already known. Thus, after writing: “It seems highly probable that the rate of mutation is influenced by environmental conditions, as Plough has shown in the case of the pomace-fly, and it is probable that environment has played a large part in the rate of evolution,” he adds: “On the other hand, the evidences against the inheritance of the effects of use and disuse are so strong that one hesitates to invoke their aid.” We submit, however, that the rôle of a changeful environment in affording mutational stimulus has very little to do with its rôle in imprinting modifications. We agree, all the same, with Prof. Conklin that there is no reason for supposing that the formulation of the factors in evolution is approaching exhaustiveness. Ætiology is still a young science.