198 
ORIGIN OF LIFE IN AMERICA 
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 faunal 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. 194—195. 
t Schuchert, Charles, “ Paleogeography of North America.” 
