Their means of passing a wide expanse of sea are far more restricted.

Their distribution has been more accurately studied, and we possess a much more complete knowledge of such groups as mammals and birds in most of the islands, than we do of the plants.

It is these two classes which will supply us with most of our facts as to the geographical distribution of organized beings in this region.
The number of Mammalia known to inhabit the Indo-Malay region is very considerable, exceeding 170 species.

With the exception of the bats, none of these have any regular means of passing arms of the sea many miles in extent, and a consideration of their distribution must therefore greatly assist us in determining whether these islands have ever been connected with each other or with the continent since the epoch of existing species.
The Quadrumana or monkey tribe form one of the most characteristic features of this region.

Twenty-four distinct species are known to inhabit it, and these are distributed with tolerable uniformity over the islands, nine being found in Java, ten in the Malay peninsula, eleven in Sumatra, and thirteen in Borneo.