<286 THE THEORIES OF EVOLUTION 



to studying the various modes of isolation, geological 

 and biological, Romanes concentrated his attention on 

 physiological selection, a new scientific notion which 

 originated with him. 



In his book on "Darwin and After Darwin," Ro- 

 manes adopted many of Gulick's ideas and the two 

 theories are identical as far as the influence of geo- 

 graphical isolation is concerned. 



Isolation, whatever its nature may be, is, accord- 

 ing to the exponents of the isolation theory, a univer- 

 sal factor, more general than even natural selection, 

 a factor ranking in importance with heredity and vari- 

 ation, the other two factors of organic evolution. In 

 virtue of this principle, the transformation of a type 

 cannot occur unless interbreeding becomes impossible 

 between a part of a species and the rest of that species. 

 When no obstacles whatever hinder interbreeding over 

 the whole area inhabited by a species, changes in con- 

 ditions, however radical they may be, cannot result in 

 the appearance of any new forms. Natural selection 

 unaided will be incapable of producing a divergence 

 of characters, Darwin's statement to the contrary not- 

 withstanding. On the other hand, whenever a geo- 

 graphical barrier arises, new types will appear, the 

 more dissimilar as the isolation lasts longer, as the dis- 

 tance is greater and life conditions are more different. 



Among the gastropods of the Sandwich Islands, 

 there is an immense number of varieties, every variety 



