198 OKIGIN OF LIFE IN AMEKICA 



western faunas which was the characteristic feature of the 

 two ancient land-masses during Mesozoic times when the 

 latter were separated from one another by a wide ocean. 

 Dr. Pilsbry * once advocated the view, from a conchological 

 standpoint, that the Sierra Nevada divided North America 

 into two primary f aunal provinces, a division which he thought 

 commenced in Jurassic times with the upheaval of this moun- 

 tain range, the latter having proved insurmountable to most 

 land snails. The true autochthonous American fauna east 

 of the Sierra Nevada range seemed to him to have developed, 

 with very little or no trace of Old World influence, from an 

 early period, probably the Cretaceous. 



This -difference between the east and west is in many 

 respects less pronounced when we include Mexico in our 

 western district. Few species of the characteristically 

 eastern snail Polygyra are found in California. Further 

 south, however, in Mexico, their numbers again increase. The 

 family of perches (Percidae) are quite absent from the western 

 States. In Mexico they again appear. Among the garpikes 

 (Lepidosteus) and the Unionidae we notice a similar distribu- 

 tion, but it is not discontinuous. Their range passes gradu- 

 ally from the Gulf States into Mexico. Among other groups 

 or genera a greater discontinuity of distribution is noticeable 

 between the Mexican and the east American centres of dis- 

 persal. The tribes that are affected by desert conditions, 

 and many of them are, would find the arid regions lying 

 directly to the north of Mexico effectual barriers to dispersal 

 in that direction. But we have good reasons for the belief that 

 these desert conditions are comparatively modern develop- 

 ments, and have little to do with the origin of the distribution 

 referred to. The geological history of the south-western 

 States may throw further light on this problem. 



From Cambrian to Jurassic times the greater part of Cali- 

 fornia was raised above sea-level. Towards the end of the 

 Mesozoic Era there existed in western North America a 

 broad strip of land running north and south being bounded 

 on each side by the sea.f Mexico must have extended further 



* Pilsbry, H. A., " Check-list of Land Shells," pp. 194195. 

 t Schuchert, Charles, " Paleogeography of North America." 



