CHAP. XV.] THE NEARCTIC REGION. 115 



the Mississippi and west of the Sierra Xevada, bear but a small 

 proportion to its whole area. Again, we know that at a very 

 recent period geologically, it was subjected to a very severe Glacial 

 epoch, which wrapped a full half of it in a mantle of ice, and 

 exterminated a large number of animals which previously in- 

 habited it. Taking all this into account, we need not be sur- 

 prised to find the Nearctic region somewhat less rich and varied 

 iu its forms of life than the Palaearctic or the Australian regions, 

 with which alone it can fairly be compared. The wonder rather 

 is that it should be so little inferior to them in this respect, and 

 that it should possess such a variety of groups, and such a 

 multitude of forms, in every class of animals. 



Zoological characteristics of the Nearctic Region. — Temperate 

 North America possesses representatives of 26 families of Mam- 

 malia, 48 of Birds, 18 of Reptiles, 11 of Amphibia, and 18 of 

 Fresh-water Fish. The first three numbers are considerably less 

 than the correspoudiug numbers for the Palaearctic region, while 

 the last two are greater — in the case of fishes materially so, a 

 circumstance readily explained by the wonderful group of fresh- 

 water lakes and the noble southward-flowing river system of the 

 Mississippi, to which the Palsearctic region has nothing com- 

 parable. But although somewhat deficient in the total number 

 of its famines, this region possesses its full proportion of peculiar 

 and characteristic family and generic forms. Xo less than 13 

 families or sub-families of Vertebrata are confined to it, or just 

 enter the adjacent Neotropical region. These are, — three of mam- 

 malia, Antilocaprin^e, Saccomyidas and Haploodoutidffi ; one of 

 birds, ChamaL'idae ; one of reptiles, Chirotidaj ; two of amphibia, 

 Sirenidae and Amphiumida^ ; and the remaining six of fresh- water 

 fishes. The number of peculiar or characteristic genera is per- 

 haps more important for our purpose ; and these are very con- 

 siderable, as the following enumeration will show. 



Mammalia. — Of the family of moles (Talpida?) m'c have 3 

 peculiar genera : Condylura, Scapanvs, and Scalojjs, as well as 

 the remarkable Urotrichus, found only in California and Japan. 

 In the weasel family (jNIustelida?) we have Laiax, a peculiar 

 kind of otter ; Taxidca, allied to the badgers ; and one of the 



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