262 MAMilALS. 



^vith an intention, and an intention to do a disagreeable thing, against their present interests and 

 inclinations for a future, distant, and problematical advantage ; a thing which, I imagine, no beast 

 ever yet did, and few men. Of course I except Polar and migratory animals which are moved by 

 their instinct to travel in a particidar direction. I therefore do not adopt the idea of Bhering's 

 Straits when frozen being a serviceable bridge for non-Polar animals to cross by. 



I need scarcely repeat that I account for the occurrence of so many plants, insects, and other 

 animals in North America, which, although recognisable as American varieties of species also found 

 in Europe or Asia, cannot be separated from them as distinct species, by the hyjjothesis that, at some 

 not very distant geological period, the New and the Old World were united at theii- northern 

 extremity, and that a bridge existed not only across the Atlantic but across the Pacific. 



Prairie Dog. (Cynomys.) As the SjDermophiles are the inhabitants of mountainous and rocky 

 places, so the Prairie Dogs are inhabitants of plains ; they are intermediate between the Marmots 

 and Spermophiles, and have, by different authors, been placed in each. Two species have been 

 described, but there are probably one or two more. They are North American, and their range 

 is extensive. The common Prairie Dog is fomid over the entire extent of the region between the 

 Missouri River and the Rocky Mountains. It has not hitherto been recorded as occurring north of 

 the United States lines. Southwards it extends to the Rio Grande, as far as the Presidio del 

 Norte, in 30° N. Lat. It is not probable, however, that it goes so far south as Matamoras, as it is 

 not noticed by Dr. Berlandier in his notes on the zoology of that region. 



Marmot. (Arctomys.) (Map. 90.) These are the largest of the Squirrel family, some of them 

 being not very far behind the Beaver in size. The number of species does not exceed seven, 

 three of them inhabiting the Old World, the rest the New. The Arctojiys bobac of Europe 

 and Asia, and A. monax of North America, are the best known ; the range of the former 

 stretches from Switzerland to Kamtschatka, and that of the latter across North America, from the 

 Atlantic to the Pacific, or rather almost to the Pacific. Such a sjjace as lies between Switzerland 

 and the Atlantic in Europe also separates A. monax from the Pacific in America, its place there 

 being taken by A. fi.aviventer. Another species, A. pruinosus, which occurs on a small tract 

 of coimtry lying on the borders of the Rocky Mountains, between the Columbia and Eraser's River, 

 may perhaps also be found in Asia. Middeudorff* says that a large species in Kamtschatka 

 exactly corresponds with it, and Baird seems inclined to adopt his view.f 



Fossil remains of an extinct species have been found in the volcanic alluvium of Auvergne. 



The Spermophiles and Marmots would, according to my hypothesis, be still more recent 

 descendants from, the Squirrels. They are entirely northern species, and as they are more largely 

 represented in America than in the Old World, the chances are in favour of their having come 

 into existence in North America, and spread from thence into the Old World by Asia. 



* MiDDENDORFF, " Siberiscbe Reise." *' 



+ Baird, loc. cit. p. 347. 



