430 
was of no moment for the Westcoast of Sumatra and the South 
coast of Java. Here, as appears from the chart on page 4i1, the 
coast became only a few kilometers broader, the rivers lengthening 
in correspondence with it, which of course had no bearing on their 
fauna. On the other hand, that influence must have been very great 
elsewhere. A river discharging itself into the Java-, or the South 
China Sea, had to cut its way, when these seas were dried up, 
into the new land, in order to find a new outlet in the retiring sea; 
it had to receive newly formed affluents, which had to drain the 
newly shaped land. But, what is of still greater importance, is that 
two rivers, which are now separated, were mutually combined or 
formed part of a larger river-system. 
Geology teaches us how the riversystem of Holland and Germany 
in the recent past differed from what they are now. How eg. the 
Thames was a branch of the Rhine, how the Seheldt flowed in a 
different direction, how a large stream, which flowed through Germany 
from East to West, united the now separated Vistula, Elb and Weser. 
In such a way the Mussi of Hast Sumatra may have been an 
affluent of a large river debouching into the China Sea, which also 
may have received the Kapuwas, discharging itself at the West coast 
of Borneo and presumably also continental Asiatic affluents. May be 
another river system emptied itself through Sunda Strait into the 
Indie, and transported besides the rainwater that fell on the land 
of the dry Java Sea, also the water of the rivers that in former 
times discharged themselves into it. 
If there is a nucleus of truth in these speculations, we may suppose 
that some of it must be visible in the present-day fauna. 
Let us suppose that the Kapuwas of West-Borneo formed, in the 
pleistocene, part of a riversystem, to which also belonged the Mussi 
of Sumatra. This would have occasioned an interchange of the 
fauna and mutual enrichment. But then this must be noticeable in 
a considerable faunistic similarity of these rivers that are now separated 
and have each an embouchure of their own. 
The Mahakkam (Kutei) of the East coast of Borneo must behave 
quite differently. 
This large stream, flowing into Macassar Strait was in no way 
affected by a decrease of 70 m. in the depth of this strait, whose 
depth amounts to some 1000 meters. It remained what it was, 
though its lower course was lengthened by several kilometers; no 
supply of water from other rivers, neither a change, nor an enrichment 
of its fauna could be expected. 
The soundness of this reasoning, therefore, would be best testified 
