As the volcanic belt of Java and Sumatra increased in activity, more and more of the land was submerged, until first Borneo, and afterwards Sumatra, became entirely severed.

Since the epoch of the first disturbance, several distinct elevations and depressions may have taken place, and the islands may have been more than once joined with each other or with the main land, and again separated.

Successive waves of immigration may thus have modified their animal productions, and led to those anomalies in distribution which are so difficult to account for by any single operation of elevation or submergence.

The form of Borneo, consisting of radiating mountain chains with intervening broad alluvial valleys, suggests the idea that it has once been much more submerged than it is at present (when it would have somewhat resembled Celebes or Gilolo in outline), and has been increased to its present dimensions by the filling up of its gulfs with sedimentary matter, assisted by gradual elevation of the land.

Sumatra has also been evidently much increased in size by the formation of alluvial plains along its northeastern coasts.
There is one peculiarity in the productions of Java that is very puzzling--the occurrence of several species or groups characteristic of the Siamese countries or of India, but which do not occur in Borneo or Sumatra.