292 NATURAL SCIENCE. Oct., 1893. 



The Southern Sub-region is composed of Mr. Allen's " Warm 

 Temperate Sub-region," and the fragmentary bits of borderland to 

 the south, which he assigns to the " American Tropical Realm." For 

 this Sub-region " Sonoran," as used by Dr. Merriam, seems to be a 

 good and appropriate term, much preferable to Mr. Allen's " Warm- 

 Temperate," which is quite indefinite. The Sonoran Sub-region of 

 the Nearctic Region, as we propose to call it, contains a much smaller 

 Palaearctic element than the Canadian Sub-region. Nevertheless, 

 Bison, TcDiilas, Spermophilus, Arvicola, and others, are undoubtedly 

 Palaearctic types. On the other hand, Antilocapra, Cynomys, Sigmodon, 

 Oiyzomys, Neotoma, Thovwniys, Dipodomys, Scalops, Urotrichus, and 

 Bnssai'is, with their allies, constitute a strong endemic force. But 

 what principally differentiates the Sonoran from the Canadian Sub- 

 region is the presence in it of a lot of intruders from the Neotropical 

 Region — such as Didelphys, Dicotyles, Cariacus, Tatusia, Nasua, and 

 Molossiis, which have nothing to do with the autochthonous fauna of 

 North America. On the whole, the Nearctic Mammal-fauna may be 

 defined as having a strong Palaearctic basis mixed up with endemic 

 elements, and invaded largely on its southern frontiers by Neotropical 

 immigrants. The presence of these Neotropical immigrants serves 

 chiefly to distinguish the Sonoran from the Canadian Sub-region, 

 though, as Mr. Allen has shown, there is also a material difference 

 in the endemic forms peculiar to the respective Sub-regions. 



