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CHAP. XV.] THE NEARCTIC REGION. 115
the" Mississippi and west of the Sierra N^evada, bear but a small
proportion to its whole area. AgOTQ, we know that at a very
recent period geologically, it was subjected to a very severe Glacial
epoch, which wrapped a fuU 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 tlie Nearctic Region.—Temperate
North America possesses representatives of 26 families of Mam-
malia, 48 of Birds, 18 of Eeptiles, 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 Palaearctic region has nothing comparable.
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 Vertebrata are confined to it, or just
enter the adjacent Neotropical region. These are,—three of mam-
malia, Antilocaprinse, Saccomyidse and Haploodontidae ; one of
birds, Chamseidse ; one of reptiles, Chirotidae ; two of amphibia,
Sirenidse and Amphiumidse ;
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 wlQ show.
Mammalia.—Of the family of moles (Talpidge) 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 (Mustelidse) we have Latax, a peculiar
kind of otter; Taxidca, allied to the badgers; and one of the