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can be brought forward is not so cogent as in the case of the fish- 
fauna of the Kapuwas and the Mahakkam. W hen studying the problem, 
difficulties will crop up. 
First of all the elements constituting a fauna are not of the same 
age. Older and younger strata occur with various possibilities of 
distribution. 
Besides this historical factor, there are also various biological 
factors of different nature: Even though a species be induced to 
migrate to other quarters and may, by doing so, possibly enlarge 
its habitat, it can avail itself of this opportunity only when the 
conditions of life in the new abode meet the requirements of the 
species. — Furthermore, the question arises whether perhaps other 
influences in the ice-period affected the countries concerned or part 
of them. The fauna of Java e.g. has ever afforded special difficulties 
for the zoogeographers. It has already been alluded to in this paper 
that Java has long since been supposed to have behaved differently 
from Sumatra and Borneo, and consequently, also differs in its fauna. 
An attempt to account for this has been made by assuming that 
Java was the first of the islands to detach itself from the land- 
complex that united them. — But the faunistic peculiarities of Java 
may also have resulted from the occurrences consequent on the 
formation of the enormous range of volcanoes that runs through the 
island from West to Hast; the products of their eruptions (ashes, 
mudstreams, and the like) may also have influenced the fauna 
directly, or indirectly by modifying the climate (through intercepting 
the sunlight by the suspended dust of ashes scattered through the 
air, or by profuse rainfall). 
Such questions will encumber the application of the CROLL-PENCK 
theory to the study of the distribution of animals in the Indo-Austra- 
lian Archipelago, but a good many of these problems will admit of. 
solution. For this theory offers a welcome basis for a number of 
hypotheses regarding former land-connections between the now 
separated islands, brought forward by zoogeographers to explain the 
facts observed. Moreover it clarifies our ideas with regard to the 
time at which the supposed land-connections originated. 
The Great Sunda Islands have been discussed above. 
But Penck’s theory also throws a new light upon the eastern half 
of the Archipelago. Here the distribution of animals led to the 
hypotbesis that New-Guinea, together with the Aru Islands, Waigeu 
and the neighbouring smaller islands, formed one land-mass, that 
was connected with North Australia. 
Those lands are now separated by shallow straits and a shallow 
