64 ORGANIC EVOLUTION 



snails being transported by birds or by some different means 

 other than their own powers of locomotion. Gulick, who 

 first called attention to the importance of segregation as a 



FIG. 10. Map of Oahu, one of the Hawaiian Islands. 



factor in evolution, was led to his conclusions by his study 

 of the remarkably restricted range of each of the many 

 species of land snails in these Oahu mountain gorges. 



The difficulties in the way of migration over great dis- 

 tances must tend toward segregation among both plants and 

 animals. The individuals at the extremes of the area occu- 

 pied by any species cannot intercross directly unless the area 

 be very limited in extent or their powers of migration very 

 considerable. And even the birds, whose powers of migra- 



