2 Mutations and Evolution. 



and to some extent on the direct effect of environment. But 

 Darwin's followers at the end of the nineteenth century, having 

 eliminated other factors, were endeavouring to show that natural 

 selection was sufficient to account for all the then known 

 phenomena of variation and adaptation. In the meantime 

 palaeontologists and anatomists, particularly the American School 

 of Neo-Lamarckians, had kept aJive the Lamarckian factor. 

 Following that period, the striking discoveries in connection with 

 mutations and Mendelian inheritance opened up a new outlook. 



Darwin made no clear distinction between continuity and 

 discontinuity in variation or inheritance, although his language 

 sometimes clearly implies that he had observed one thing or the 

 other. In some cases infinitesimal variations were apparently in 

 his mind in writing, but when he refers to " individual differences" 

 it is usually clear that he is citing what we should now call small 

 or parvigrade (Poulton) mutations. In one aspect then, mutation 

 as an evolutionary factor represents a refinement and an 

 increased precision in the application oty Darwin's theory 

 of natural selection. That he recognized the occurrence of 

 variations which were not necessarily subject to natural selection 

 is clear from such passages as the following in the 

 11 Origin " : " I am inclined to suspect that we see, at least in some 

 of these polymorphic genera (such as Rubus, Rosa, Hieracium and 

 fossil Brachiopods), variations which are of no service or dis- 

 service to the species, and which consequently have not been seized 

 on and rendered definite by natural selection " (p. 33). 



If we go but a step further, and assume that these variations 

 are followed by others in the same population with elimination of 

 some steps by selection ; or if we assume the immediate origin of 

 new specific types through single mutations, and their gradual 

 spread from the centre of origin, then we are using the mutation 

 theory of de Vries as it is widely applied at the present time. 

 Darwin's objections to "sudden and considerable deviations of 

 structure " as material for evolution, were based largely upon his 

 observations of wide saltations and monstrosities. They included 

 the well-known argument of swamping through crossing, which has 

 since been negatived by the Mendelian discoveries. 



It cannot then be said that the present Mendelian-mutationist 

 attitude in its general aspects represents more than a refinement 

 of Darwin's main thesis, based upon greatly increased knowledge 

 of variation, inheritance, and cell structure ; and a restriction of 



