SEGREGATION 63 



would expect from these conditions, that the Siberian 

 and western American faunas and floras, while having 

 many forms which are closely similar because of common 

 descent, are still distinct, having very few species in com- 

 mon. (Of common genera, of course, there are many.) 

 Natural selection, aided by segregation, has had time to 

 produce great changes. 



Diversity in soil conditions produces segregation among 

 plants, and local differences in food conditions thus aris- 

 ing must cause segregation among animals, different 

 groups of a single species being found in the separate 

 localities where the suitable conditions of soil or food 

 exist. 



One of the finest examples of extreme segregation within 

 a limited area is furnished by the land shells of Oahu, one of 

 the Hawaiian Islands. Along the northeastern shore of the 

 island is a high mountain range whose sides have by erosion 

 been cut into deep valleys (Fig. 10) with high and steep 

 ridges between. The soil in the lower ground of each valley 

 is rich and bears a profusion of tropical trees, shrubs, ferns, 

 and other plants. The tops of the main ridge, however, and 

 also the tops of the lateral ridges, are barren, being denuded 

 of their soil by the heavy rains. Several genera of land 

 snails, which feed upon the foliage of the trees, shrubs, and 

 herbs, are very abundant along this whole series of valleys, 

 and it is interesting to observe that each of the several 

 species (or varieties ?) of snails is confined to a single valley 

 or to two or three adjacent valleys. Their proper food and 

 the necessary shade being absent on the tops of the ridges, 

 the snails do not cross from one valley to the next. Such 

 spreading as has occurred has probably been due to the 



