28 ORIGIN OF LIFE IN AMEEICA 



small carnivores and a variety of the black bear (Ursus 

 americanus).* 



The small mouse-like creatures belonging to the genus 

 Synaptomys are aptly called " lemming-voles " because, ex- 

 ternally like lemmings, their teeth approach those of voles. 

 When Dr. Merriam undertook his revision of the species of 

 Synaptomys ,f seven kinds were known to him. A few more 

 have since been discovered. The genus is entirely confined to 

 the North American continent, and all the species except two 

 have a limited range in the boreal region. One of the latter 

 (Synaptomys cooperi) is found from Massachusetts westward 

 to Minnesota and southward to North Carolina, the other is 

 peculiar to Mount Washington. Almost all the other species 

 are confined to Canada and Alaska. It does not seem, there- 

 fore, as if Synaptomys innuitus, which inhabits the coast of 

 Labrador, were a recent immigrant from the south. On the 

 contrary, the genus Synaptomys, being almost confined to the 

 north, is probably of boreal American origin, one adaptable 

 species having advanced far southward along the east coast 

 of America. 



The next two species alluded to, Microtus enixus and M. 

 pennsylvanicus, are true voles, though the last-named animal 

 is generally known in the States by the name of " meadow- 

 mouse." Both of them belong to a sub-genus which is 

 verj' widely spread in Europe, Asia and boreal North 

 ^ America. f Some migrations across ancient land connections 

 ' must have taken place, no matter whether we assume that the 

 sub-genus is of Old World or New World origin. It only 

 ' remains for us to determine whether Microtus crossed the 

 North Atlantic land bridge or the one supposed to have been 

 situated at Bering Strait, assuming that there was such a one. 

 Professor Tullberg thought the members of the genus Microtus 

 had travelled to America in Pliocene times by means of the 

 first land bridge. § This view does not appear to me pro- 

 bable, because the family is entirely absent from Ireland, 



* Bangs, 0., " Mammals of Labrador." 



t Merriam, 0. H., " Revision of Synaptomys." 



t Bailey, V., " Revision of Microtus." 



§ Tullberg, Tycho, " System der Nagetiere," p. 499. 



