OTHER THEORIES OF GERMINAL SELECTION 411 



is almost identical with the Nesomimus of little Culpepper, a 

 speck nearly a hundred and fifty miles to the northwest. 



" . . . if we conceive that the ancestors of the birds on Wenman, 

 Tower and Indefatigable were identical, and gradual subsitlence 

 separated the three, causal variation would go on with a slightly 

 greater emphasis on one character or another, and with no possi- 

 bility of indication of specifically separate relationship. . ." 



Such is the action of isolation upon the course of evolution. 

 Merely the separation of individuals occupying different positions 

 in the range of variation of the species is enough to account for 

 differences in succeeding generations in the separate localities. It 

 makes no difference whether the characters concerned are impor- 

 tant or indifferent; the mere fact that they are a part of the 

 heritage of the species is enough to make them a source of future 

 difference in the isolated races. The distribution of song sparrows 

 in North America is a similar case which has been expressed 

 graphically in Figure 209. 



Although Darwin placed little emphasis upon this theory, 

 recently scientists have recognized that it is certain to be a potent 

 factor in the shaping of species. It almost seems axiomatic, in 

 view of our knowledge of geological changes, the possibility of 

 accidental dispersal beyond normal barriers, and the ordinary 

 range of variation, that this should be the case. Quoting Newman, 

 "If natural selection may be said to be the prime factor in pro- 

 ducing adaptations, isolation may be said to be the prime factor 

 in species differentiation, guided only within moderate limits by 

 natural selection." 



Common Objections to Germinal Theories. In conclusion we 

 should note several characteristics of these theories of evolution 

 which place them on an equal basis with the Lamarckian theories 

 to be treated in the next chapter. 



1. No theory of germinal evolution accounts for the origin of 

 variations. 



2. Only the mutation theory offers an adequate explanation of 

 change beyond the isolation of pure lines. 



3. Specific change has been accomplished very gradually in so 

 many cases that the mutation theor^^, in spite of its obvious value, 

 seems to be limited in application. 



Summary. The limitations of natural selection as an explana- 

 tion of the origin of species were clearly indicated by Johannsen's 



