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122 ZOOLOGICAL GEOGRAPHY. '
[part hi.
any other. Though not wy rich, and having many disadvan-
tages of climate and of pl^sical condition, it is yet sufficiently
well 'characterized in its zoological features to rank as one of
the well-marked primary divisions of the earth's surface.
There is one other consideration bearing on this question
which should not be lost sight of. In establishing our regions
we have depended wholly upon their now possessing a sufficient
number and variety of animal forms, and a fair proportion of
peculiar types ; but when the validity of our conclusion on these
grounds is disputed, we may supplement the evidence by an
appeal to the past history of the region in question. In this
case we find a remarkable support to our views. During the
whole Tertiary period, North America was, zoologically, far
more strongly contrasted with South America than it is now
while, during the same long series of ages, it was always clearly
separated from the Eastern hemisphere or the Palsearctic region
by the exclusive possession of important families and numerous
genera of Mammalia, as shown by our summary of its extinct
fauna in Chapter VII. Not only may we claim North America
as now forming one of the great zoological regions, but as having
continued to be one ever since the Eocene period.
Insects.
In describing the Palsearctic and Neotropical regions, many of
the peculiarities of the insect-fauna of this region have been
incidentally referred to ; and as a tolerably fuU account of the
distribution of the several families is given in the Fourth Part
of our work (Chapter XXI.), we shall treat the subject very
briefly here.
Lepidoptera.—The butterflies of the Nearctic region have
lately been studied with much assiduity, and we are now able
to form some idea of their nature and extent. Nearly 500
species belonging to about 100 genera have been described;
showing that the region, which a few years ago was thought to
be very poor in species of butterflies, is really much richer than
Europe, and probably about as rich as the more extensive Palse-
arctic region. There is, however, very little speciality in the
;