CHAPTER XVIII 



Isolation 



Geographical isolation and physiological isolation. — Moritz 

 Wagner's theory. — Wallace on the distribution of American 

 butterflies and island fauna. — Guliek and Romanes. — Two 

 modes of isolation. — D. S. Jordan. — Physiological selection. 

 — Vernon's theory of reproductive divergence. — Discussion. 



AMONG the so-called auxiliary systems, which 

 accept either the Lamarckian or the Darwinian 

 premises, but modify those fundamental conceptions 

 by laying special stress on some special factor, we 

 must mention in the first place the theories of isola- 

 tion, which were the first in date, as they simultane- 

 ously appeared with the formulation of the Darwin- 

 ian doctrine. 



Their basic idea is that fortuitous variations can not 

 give rise to a distinct species, unless some obstacle pre- 

 vents the individuals affected by those variations from 

 mating with constant individuals, unless, in other 

 words, the varied individuals are isolated from the 

 constant. 



Isolation may be due to several causes: migration 

 may lead a part of the species into a different region ; 

 a geographical barrier may divide the region inhab- 



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