CHAP. XV.] 
THE NEARCTIC REGION. 
115 
the Mississippi and west of the Sierra Nevada, 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 
in its forms of life than the Palsearctic 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 corresponding numbers for the Palsearctic 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 Pahearctic region has nothing com- 
parable. But although somewhat deficient in the total number 
of its families, this region possesses its full proportion of peculiar 
and characteristic family and generic forms. No less than 13 
families or sub-families of Yertebrata are confined to it, or just 
enter the adjacent Neotropical region. These are, — three of mam- 
malia, Antilocaprinse, Saccomyidas and Haploodontidse ; one of 
birds, Chamaeidas ; one of reptiles, Chirotidae ; two of amphibia, 
Sirenidse and Amphiumidae ; 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 (Talpidse) we have 3 
peculiar genera : Condylura, Scapanus , and Scalops , as well as 
the remarkable Urotrichus , found only in California and Japan. 
In the weasel family (Mustelidae) we have Laiax , a peculiar 
kind of otter; Taxidca , allied to the badgers; and one of the 
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