280 COOK 



onomic recognition as species, but this is a mere incident of 

 evolution, not an actuating cause nor a necessary effect. 



The recognition of heterism or diverse, alternative descent, 

 and the frequent development of sexual and other specializations 

 of heterism inside specific lines, shows that the subdivision of 

 species is to a very small extent, if any, the direct result of evo- 

 lutionary advance. Not only can diverse characteristics exist 

 inside specific lines, but it is an advantage to maintain just such 

 heterogeneity. The only condition in which heterism would 

 directly conduce to the formation of a new species would be that 

 of alternative characters which hindered interbreeding. It is 

 conceivable, for example, that a species might contain at the 

 same time variations both toward earlier and later flowering, 

 and that, instead of counteracting each other, both tendencies 

 might become gradually more accentuated. The incidental 

 result would be that interbreeding would cease and two separate 

 groups would become established. 1 In such a case it might 

 well be claimed that evolution had directly resulted in the multi- 

 plication of species, but it would still be true that it had done 

 so only by means of segregation, and would show only that 

 evolution might result in segregation, not that segregation is a 

 factor in evolution, as often supposed. Isolation is an important 

 consideration in phylogeny or historical biology, which under- 

 takes to tell why the species are in the places we find them. 

 But isolation and species-subdivision have only a remote aud 

 incidental connection with evolution ; they do not cause the pro- 

 gressive change. 



The confusion of evolution with speciation has greatly impeded 

 the progress of evolutionary science by withdrawing attention 

 from the real issues to relatively unimportant considerations. 

 It has misled many students of evolution into the belief that 

 isolation or segregation is an important factor of evolutionary 

 progress, whereas its influence is negative rather than positive. 

 The selection doctrine of Darwin and the mutation doctrine of 

 De Vries are both theories of speciation rather than of evolution. 



^he hickory-borer {Clytus f ictus) and the locust-borer {Clytus robinice) are 

 very similar species, and the females are quite indistinguishable. The perfect 

 insects of the former emerge however, in June, those of the latter in September. 

 See Packard, A. S., 1880, Guide to the Study of Insects, p. 497. 



