Mutations and Evolution. 
27 
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 alive 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 lie 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 ofy 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 
“ Origin ”: “ I am inclined to suspect that we see, at least in some 
of these polymorphic genera (such as Rubus, Rosa, Hievacium 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 
