178 EVOLUTION 



of the land-snail Cerion in the Bahamas, and 

 Gulick records over 200 species of the land- 

 snail Achatinella in the various valleys of the 

 Sandwich Island Oahu. 



Many evolutionists Wagner, Weismann, 

 Gulick, Romanes, Jordan, and others have 

 worked at the idea of Isolation, as a directive 

 factor in evolution; and Romanes maintained 

 that it was a sine qua non in the origin of new 

 species. The term must not be thought of 

 in any narrow sense; it includes all the means 

 which restrict the range of intercrossing 

 within a species : geographical barriers, such 

 as arise when a peninsula becomes an island; 

 temporal barriers, such as arise when the 

 members of a species reach sexual maturity 

 at different tunes of year; habitudinal barriers, 

 when a species splits into two or more castes 

 with different habits of life; physiological 

 barriers, such as arise by some variation in 

 the reproductive organs ; and psychological 

 barriers, which rest on profound antipathies. 



What probably happens is this : a success- 

 fully vigorous and adaptive species spreads; 

 the several contingents become isolated from 

 one another ; and, if different variations 

 spring up in several or all of the contingents, 

 then, other things being equal, isolation 

 will favour the origin of distinct species. " I 



