202 
INDIAN FRESH-WATER FISHES. 
As a rule it would seem advisable to employ for 
stocking ponds such species as naturally thrive 
best in the same part of the country, as these are 
more likely to succeed than species imported from 
other districts. It is not easy to tell beforehand 
what conditions are necessary for any particular 
species to live and breed, and especially to thrive, 
as they may be able just to exist without attaining 
the size or quality that they would do in their 
natural location. In places where particular species 
do not occur naturally, it would seem probable that 
the conditions are not favourable to them, unless 
any special reason is apparent for their non-appearance. 
We are led to this inference by finding so many 
species in places that happen to be suited to them 
all over the Indian continent, though not to be found 
in intermediate localities, and also by the paucity 
of cases in which any species is confined only to one 
river-system or district. 
I am alluding here only to Indian species. There 
might be among the Himalayan ranges many streams 
admirably adapted to European fish, such as trout, but 
no one would expect to find them there for this 
reason only. But if it were an Indian fish in 
question, I should say that the fact of any par- 
ticular species not being found in a stream would be 
a good primd-facie reason for concluding that the 
stream was not adapted to the requirements of that 
species, and that even if it were introduced from 
elsewhere it would not get on well therein. 
