ISOLATION 283 



ited by a species into two sections which no longer 

 intercommunicate (geographical isolation) ; finally a 

 variation may arise which creates a physiological hin- 

 drance to reproduction (physiological isolation). 

 Most authors consider the influence of geographical 

 distribution and of physiological variation simultane- 

 ously. 



Moritz Wagner, a famous German explorer and 

 naturalist, was the first to formulate the theory of 

 geographical isolation. His travels and observations 

 in America, Asia and Africa, led him to the conclu- 

 sion that, while natural selection might modify spe- 

 cies, it could never differentiate species, that is, pro- 

 duce new species. Geographical isolation alone can 

 produce new forms which become constant. A con- 

 tinuous isolation of this kind never fails to bring about 

 a differentiation, for the modifications due to a change 

 in habitat are necessarily perpetuated through exclu- 

 sive interbreeding of individuals having varied along 

 the same line. 



Changes in habitat are of very common occurrence, 

 for overcrowding and scarcity of food create in ani- 

 mal species a necessary tendency to migrate, to spread 

 out more and more over the face of the globe. 



Wagner's first enunciation of his theory was made 

 in 1868 in a paper entitled: Die Darwinsche Theorie 

 und das Migrations gesetz der Organismen, and until 

 his death in 1887 he steadily fought for his views as 



