MUTATIONS AN'I) I'A'OLl'TIOX. 3OI 



variations should confer sonic advantage, the individuals 

 possessing them may be expected to l)cconie more numerous, and 

 in time we may get an assemblage, all the members of which 

 will have undergone the transformation. If the character is 

 distinctive enough it will be held that specific diversity or evolu- 

 tion has taken place, though without any intensification of the 

 original mutation apart from a numerical one. Writing from 

 the mutation standpoint, Prof. T. H. Morgan* is quite clear as 

 to the conditions under which evolution may be expected to take 

 place : " The mutation process rests its argument for evolution 

 on the view that among the jxjssible changes in the genes, some 

 combinations may happen to produce characters that are better 

 suited to some place in the external world than were the original 

 characters." Variations having arisen as a result of genetic 

 changes, natural selection then determines which will survive, 

 or the course which evolution will pursue. But the genetic 

 changes themselves are the basis of evolution, and the elimination 

 of the unfavourable is merely a directive influence. Mutations 

 give the material of evolution and natural selection determines 

 its course. 



From the above it would appear that transformation of a 

 race can take place only when the mutations are of advantage 

 to the individual; the vast multitude of indifferent changes which 

 produce diversity are not the materials upon which the course 

 of evolution depends. Such a view, however, takes no account 

 of the possibility of a dominating recurrence of the same muta- 

 tion or ccmbination of mutations within a race apart from any 

 advantage considerations. The attitude can perhaps be under- 

 stood, for hitherto but scanty experimental evidence has been 

 forthcoming in favour of a general recurrence of any particular 

 mutation. A certain klegree of repetition is sometimes observ- 

 able, as several workers have pointed out in Drosophila. and as 

 de Vriest has shown for the peloric toad-flax, but little to indi- 

 cate a determinate dominance of any one change over the others. 

 It mu.st be admitted that if a germinal change is altogether 

 dependent upon some purely fortuitous factorial irregularity, 

 there can be little ground for expectation that it will occur again 

 and again. But if it should be determined by some intrinsic 

 condition or influence connected with the germ plasm of the race, 

 a change having been effected once affords a strong likelihood 

 of its repetition in other individuals, and it may even come to 

 represent a dominating influence and ultimately transform the 

 whole assemblage. Without being of any selection value, a muta- 

 tion may recur so often that in time it comes tp replace the 

 original form, and we may get the gradual evolution of a new 



* "Evolution by Mutation," "The Scientific Monthly." vol. 7, July, 



t "Species and Varieties: The-ir Origin by Mutations," London, 1905, 

 r. 479- 



