1903.] Allen, Mammals from Northeast Siberia. 183 



circumpolar species are resolvable into a number of well- 

 marked forms, which occupy definite geographic areas, and 

 are characterized by easily recognized differences. That 

 there is, nevertheless, a close interrelationship between the 

 forms of boreal mammals inhabiting the two continents is 

 beyond question, a relationship so intimate that it could 

 only have been brought about by a former land bridge 

 connecting the two areas, the existence of which in com- 

 paratively recent time, geologically speaking, is generally 

 conceded, if not practically demonstrated. 



It is thus probable that most of the more northern types 

 of mammal life on the two continents are the slightly modi- 

 fied descendants of types which formerly had a continuous 

 circumarctic distribution, which have become slowly differ- 

 entiated, probably mainly since the disruption of the former 

 land connection at Bering Straits. To this category belong the 

 whole of the ursid, canid, felid, and mustelid series (excepting, 

 of course, the mephitine phylum), and such genera as Cervus, 

 Rangifer, Parolees, Ovis, and Ovibos, and possibly Bison 

 among Ruminants, and Sorex, Evotomys, Microtus, Dicros- 

 tonyx, Lemmus, Sciuropterus, Sciurns, Eutamias, Citellus, 

 Arctomys, Lepus, and Ochotona among the Insect ivores and 

 Rodents. These types are so wide-spread and so diversified 

 on both continents that it is hard to suppose that any of them 

 owe their presence in America to any very recent immigra- 

 tion from Asia, or the reverse. Possibly, however, Cervus, 

 Bison, and Eutamias may have been direct contributions from 

 one continent to the other, the former from Eurasia to Amer- 

 ica, and the two latter from America to Eurasia, judging by 

 their present relative representation in the two areas. 



But the cases especially in point in the present connection 

 are the occurrence along the Siberian and Kamchatkan coasts 

 of types distinctively American. These are a speties of 

 weasel (Putorius pygmcsus) closely related to Putorius rixosus 

 of arctic and subarctic America, and only remotely related 

 to any Eurasiatic species; a spermophile (Citellus buxtoni) 

 closely related to the Citellus (=Spermophilus auct.) parryi 

 group of boreal America, but only remotely related to any 



