DISPERSAL OF NORTH AMERICAN BIOTA. 65 



Pcrdita are very abundant, and as Prof. T. D. A. Cockerell informs 

 me, are very characteristic, only a few species occurring east of the 

 Great Plains (Cockerell, '98). The beetles of the family Tene- 

 brionidce are quite abundant. The ant-lions, Myrmcleonidce, here 

 reach their greatest development in variety and abundance. The 

 fish fauna is limited and peculiar ; those of the Rio Grande have 

 Mississippi River affinities, while those of the Colorado River 

 show much endemism, as shown by Meek ('03). Of its 32 species, 

 only 10 are known to occur elsewhere. 



Migration and Dispersal Routes. As only a small part of this 

 southwestern center was invaded by the Glacial ice its geographic 

 position has been relatively stationary. Since the Ice Age, how- 

 ever, there has been considerable overflow to the north. Start- 

 ing in the southwest this biota has been spread northward along 

 each side of the Rocky Mountains, and has invaded an arid re- 

 gion, a condition to which it was evidently well adapted. Even 

 glaciated portions of British America were reached on each side 

 of the mountains by these hardy forms of life. 



Other plants and animals have spread from here into the south- 

 east, where on account of its varied conditions of life they have 

 been able to flourish. This, for example, is seen in the case of 

 yucca, lizards, and pocket gophers. These forms have been 

 able to find favorable arid local conditions in the southeast, as in 

 the pine barrens and on dry hillsides, etc. 



These arid types find their eastern extensions upon the dry 

 uplands interdigitating with the southeastern types which fre- 

 quent the- moist valleys. They reach their extreme eastern ex- 

 tension, in abundance and in association, upon the prairies of Wis- 

 consin, Illinois, and northern Indiana. But with the clearing away 

 of the forests this eastward advance has been greatly hastened. 



The aquatic life of this center has communicated with the Mis- 

 sissippi River, as shown by the Rio Grande fish fauna, but that 

 of the Colorado River has been isolated to an exceptional de- 

 gree, and has developed a remarkable individuality. 



3. SOME FACTORS IN BIOTIC INTERPRETATION. 

 In a previous paper (Adams, 'oi) the writer has discussed the 

 relation of the baseleveling processes upon habitat differentiation 



