431 
by a comparative investigation of the Kapuwas and the Mahakkam. 
Material for comparison could be procured by the fish-fauna, this 
being best known. 
In selecting our fish-material we had to shift critically, and to 
make many restrictions. We had to exclude marine immigrants, 
indeed all so-called anadromous and catadromous fishes; secondly all 
fishes living in brackish water; only those species could be used for 
which seawater is an insurmountable barrier. For when at the close 
of the ice-period, which for the sake of convenience we will con- 
sider to have been a continuous period, the water resulting from 
the melting ice and snow gradually raised the level of the oceans, 
the seawater in the neighbourhood of the large river-mouths of 
Borneo and Sumatra will have been of a brackish nature prior to 
the present condition of the sea. At that time it was, then, possible 
for fish that could stand brackish water, to migrate from one river 
into another. That possibility disappeared only when the definitive 
salinity was established permanently. 
After this shifting our working-material consisted only of two 
species of Notopterus, one Scleropages, 17 genera of Siluroids with 
39 and 37 genera of Cyprinoids with 100 species, altogether 56 
genera with 142 species. 
The reliability of our results will increase with the extent of our 
material. We will, therefore, lay stress on the full significance of 
the number of 142 species. It appears from the fact that the number 
of true freshwater fishes, in the restriction given above, which excludes 
marine immigrants, amounts to only 60 species for the vast land- 
complex that comprises the Netherlands, Belgium, Germany and the 
Danubian countries as far as the Black Sea. 
We have tabulated below our material taken from the Kapuwas 
and the Mahakkam, and have added those species that occur also in 
the rivers flowing into the Java Sea at the South Coast of Borneo. 
The table also contains those species that are found in East Sumatra, 
in Java and in rivers of the Asiatic continent (Malacca and Siam). 
From this we see that of the 142 species only 52 are common 
to both rivers. Of the 90 remaining species 23 belong to the Mahakkam 
and 67 to the Kapuwas. Of the 67 species that do not occur in the 
Mahakkam 55 (82°/,) are represented also in other rivers, viz. 75 °/, 
in the rivers of East-Sumatra. Only 12 species (1,8 °/,) are restricted 
to the Kapuwas, or are known from neighbouring rivers, also flowing 
into the South China Sea. | 
On the other hand the Mahakkam possesses 23 species which the 
Kapuwas lacks. But of these 23 species 17 (74°/,) are indigenous 
