difference from other regions; and we may also give lists of the
families and genera inhabiting each, with indications as to which are
peculiar and which are also found in adjacent regions. This aspect of
the study I term zoological geography, and it is that which would be of
most interest to the resident or travelling naturalist, as it would give
him, in the most direct and compact form, an indication of the numbers
and kinds of animals he might expect to meet with.[3]

The keynote of the general scheme of distribution, as set forth in these
two volumes, may be expressed as an endeavour to compare the extinct and
existing fauna of each country and to trace the course by which what is
now peculiar to each region had come to assume its present character.
The main result being that all the higher forms of life seem to have
originally appeared in the northern hemisphere, which has sent out
migration after migration to colonise the three southern continents; and
although varying considerably from time to time in form and extent, each
has kept essentially distinct, while at the same time receiving
periodically wave after wave of fresh animal life from the northward.

This again was due to many physical causes such as peninsulas parting
from continents as islands, islands joining and making new continents,
continents breaking up or effecting junction with or being isolated from
one another. Thus Australia received the germ of her present abundant
fauna of pouched mammals when she was part of the Old-World continent,
but separated from that too soon to receive the various placental
mammals which have, except in her isolated area, superseded those older
forms. So, also, South America, at one time unconnected with North
America, developed her great sloths and armadilloes, and, on fusing with
the latter, sent her megatheriums to the north, and received mastodons
and large cats in exchange.