6 4 



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 



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



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



J J 



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- 



