330 OKIGIN OF LIFE IN AMERICA 



of a low -lying plain. This district, moreover, is entirely 

 granitic and composed of a number of high ridges running 

 parallel in an east- westerly direction, the remainder of the 

 great peninsula being largely formed of calcareous rocks with 

 mountain ranges running in a north and south direction. 

 Between the two lies a great plain several hundred miles long 

 with a height of scarcely one hundred and fifty feet above sea- 

 level. The two mountain ranges manifestly belong to entirely 

 different systems, and the junction between the two must have 

 been a comparatively recent geological event. Mr. Eisen* 

 was so much impressed by the supposed severity of the climate 

 during the Glacial Epoch that he believed the whole Cape 

 region was at that time wrapped in snow and ice and devoid of 

 animal life. But he also contends that it must have been an 

 island and that during its rise animals and plants gradually 

 reached it from the mainland by accidental transport. That 

 the Cape Region has only recently become part of Lower Cali- 

 fornia is highly probable. To judge from the fauna and flora, 

 it must have been connected by land with some part of Central 

 America or southern Mexico, though it possesses affinities, too, 

 with Asia and the Pacific islands (compare, p. 208). Rather 

 more than half-way across the sea between the Cape Region 

 and the south coast of Mexico lies the small group of tihe 

 Tres Marias islands, and it might be argued that they 

 had once formed the connecting link between the mainland 

 and that faunistically so remarkable Cape Region of Lower 

 California. The animals and plants of these islands, how- 

 ever, although clearly showing that the islands have been 

 joined to one another and to southern Mexico, exhibit no 

 near relationship to those of the Cape Region.} Hence it 

 is probable that the faunistic and floristic affinity between 

 the Cape Region and southern Mexico is due to the fact 

 that both regions have acquired their animals and plants, in 

 more remote times, from the same source in Central America. 

 I suggested in a former chapter (p. 287) that the moun- 

 tains of Guatemala had once extended further westward. 

 Guatemala certainly seems to have been a land surface 



* Eisen, G., " Explorations in the Cape Region," p. 735. 

 t Nelson, E. W., L. Stejneger, and others, " Natural History of the 

 Tres Marias Islands." 



