mammalia, reptiles, and other great divisions; while at the same time
it helped to explain the apparent contradictions in the distribution of
land animals. Some years later he wrote:

In whatever work I have done I have always aimed at systematic
arrangement and uniformity of treatment throughout. But here the immense
extent of the subject, the overwhelming mass of detail, and above all
the excessive diversities in the amount of knowledge of the different
classes of animals, rendered it quite impossible to treat all alike. My
preliminary studies had already satisfied me that it was quite useless
to attempt to found any conclusions on those groups which were
comparatively little known, either as regards the proportion of species
collected and described, or as regards their systematic classification.
It was also clear that as the present distribution of animals is
necessarily due to their past distribution, the greatest importance must
be given to those groups whose fossil remains in the more recent strata
are the most abundant and the best known. These considerations led me to
limit my work in its detailed systematic groundwork, and study of the
principles and law of distribution, to the mammalia and birds, and to
apply the principles thus arrived at to an explanation of the
distribution of other groups, such as reptiles, fresh-water fishes, land
and fresh-water shells, and the best-known insect Orders.

There remained another fundamental point to consider. Geographical
distribution in its practical applications and interest, both to
students and to the general reader, consists of two distinct divisions,
or rather, perhaps, may be looked at from two points of view. In the
first of these we divide the earth into regions and sub-regions, study
the causes which have led to the difference in their animal productions,
give a general account of these, with the amount of resemblance to and