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mammals which have, except in her isolated area, superseded those older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.
Some of the points, such for instance as the division of the sub-regions
into which each greater division is separated, gave rise to
considerable controversy. Wallace's final estimate of the work stands:
"No one is more aware than myself of the defects of the work, a
considerable portion of which are due to the fact that it was written a
quarter of a century too soon--at a time when both zoological and
palaeontological discovery were advancing with great rapidity, while new
and improved classifications of some of the great classes and orders
were in constant progress.
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