212 DARWINISM AND HUMAN LIFE 



snail Cerion on the Bahama Islands, and GuUck 

 reports 200-300 species of the land-snail Achati- 

 nella in the various valleys of the Sandwich Island 

 Oahu. 



President Jordan has devoted some attention 

 to the occurrence of cognate or " geminate " 

 species on opposite sides of some barrier. " In 

 a general way, such species agree with each other 

 in all the respects which usually distinguish species 

 within the genus. Their difierences appear in minor 

 regards, characters of degree, or proportion — 

 traits which we may safely suppose to be of more 

 recent origin than the ordinary characters marking 

 off species within the group." As examples of 

 what are probably in some measure the results 

 of isolation, he takes the following : " Each 

 well-separated island in the West Indies has its 

 own form of golden warbler. Each island in the 

 East Indies has its own forms of reptiles, monkeys, 

 snails, and fresh-water fishes. Each island in 

 Hawaii has its own species of each genus of 

 Drepanine birds ; each forest its own type of 

 land-snails. Each of the three groups of rookeries 

 in Bering Sea has its own species of fur-seal. 

 Each section of the Isthmus of Panama has its 

 geminate species of fishes, representing nearly 

 every genus or sub-genus of the shore-water off 

 Mexico." ' 



There is considerable evidence to show that 

 isolation, with its attendant inbreeding, has played 

 an important role in human evolution, fixing and 

 intensifying and giving hereditary grip to types 

 which began their career in small communities. 



^ " Isolation as a Factor in Organic Evolution," in " Fifty 

 Years of Darwinism" (1909), p. 81. 



