14 NATURE SKETCHES IN TEMPERATE AMERICA 



mutations now appearing, as is claimed, independently of 

 variations that took place in an earlier period in the history of 

 these plants? Secondly, if species can spring into existence 

 at a single leap without the assistance of cumulative variations, 

 may they not also originate with such assistance? That varia- 

 tion does issue in new species and that selection is a factor, 

 though not the only factor, in determining results is in my 

 opinion as certain as that grass grows, although we cannot see 

 it grow. It is then after all the shght individual differences 

 that suffice for the work and are probably the sole differences 

 which are effective in the production of new species." 



Furthermore, Tower says: "Of late there has grown up, 

 since the j)ublication of De\'ries' work, a tendency to ascribe 

 to 'mutation' a far greater importance, and to a considerable 

 extent to substitute that process for all others in evolution, 

 even though De\'rics distinctly points out that 'mutation' 

 is complementary and not antagonistic to natural selection, 

 and that the two are necessary in evolution ^ — that is, 'muta- 

 tion' explains the origin of variations in evolution, and natural 

 selection their preservation." ' 



Weismann maintained that there are three principal stages 

 of selection: tliat of personal sclec-tion as held by Darwin and 

 Wallace, that of histonal selection as upheld by Roux in the 

 form of the struggle of the parts, and, finally, that of germinal 

 selection, the existence of which he endeavored to establish. 

 He called the reproductive substance the germ plasm, and he 

 maintained that this sulistance is distinct from the body and 

 the influences that modify the character of the one do not 

 necessarily modify the character of the other. These are the 

 supposed factors that cooperate to maintain the forms of life 

 constantly capable of life. It may be interesting to note that 

 the study of variation has been aided considerably by mathe- 

 matical science, that is, by the use of statistical measure- 

 ments. This is accomplished by taking a .series of hundreds 

 or thousands of individuals of a species in a state of nature and, 

 by making careful measurements of some of the more impor- 

 tant structures of the animal and reducing them to figures, 



' Investigation of Evolution in Clirysomelid Beetles of the Genus Leptino- 

 tarsa. 



