234 DARWINISM TO-DAY. 



by some authors to name the same condition), ii, in a 

 species, a number of individuals show a certain congenital 

 variation, this variation will probably be lost by 

 cross ~breeding with individuals not having it, 

 unless the individuals having it are in the ma- 

 jority or unless they become in some way isolated from the 

 others and segregated so that they will breed among them- 

 selves. By such isolation and such in-and-in breeding the 

 newly appearing congenital variations might soon become 

 established, and if advantageous be so considerably developed 

 as soon to distinguish as a variety or incipient species the 

 members of the isolated colony. With time a distinct new 

 species might result. Are there means to produce such isola- 

 tion of groups of individuals belonging to a common species ? 

 The answer to this is certainly an affirmative one. There 

 seem to be, indeed, several means of producing isolation, 

 _ . and the isolation may be variously named ac- 



Varions means 

 of effecting iso- cordingly. Undoubtedly the most important of 



these kinds of isolation, at least in the light of 

 our present knowledge, is that known as geographical or 

 topographical isolation. Isolation produced in other ways 

 may be called biologic or physiologic or sexual isolation. 

 In the case of geographic or topographic isolation the iso- 

 lated group or groups of individuals are actually in another 

 region or locality from the rest of the species, this being 

 the result of migration, voluntary or involuntary. In bio- 

 logic isolation the individuals of the species all inhabit the 

 same territory but become separated into groups by struc- 

 tural or physiological characters which prevent 

 Wagner the miscellaneous inter-breeding. The real founder 



lonnder 01 the 



theory of species- and most insistent upholder of the theory of 

 species-forming by isolation (geographic and 

 topographic isolation), was Moritz Wagner 3 

 (1813-1887), a traveller and naturalist, whose wanderings 

 and observations brought to him the conviction that while 



