122 
ZOOLOGICAL GEOGRAPHY. 
[part III. 
any other. Though not very rich, and having many disadvan- 
tages of climate and of physical 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 hearing on this question 
which should not he 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 ; hut 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 Palaearctic 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 full 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, 
Lcpidoptera . — 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 Palee- 
arctic region. There is, however, very little speciality in the 
