16 
INDIAN FRESH-WATER FISHES. 
arranged museum ; collectors who discover and de- 
scribe a new species, ought invariably to deposit 
type specimens in a museum where they will always 
be available for reference and comparison. 
The necessity for this becomes at once apparent if 
we look at a work like the British Museum Catalogue. 
Take ' the common Dace, for instance, Leuciscus 
vulgaris ; it has been described under about 25 differ- 
ent names by various authors, and these on compari- 
son all proved to refer to one and the same species ! 
Or take an Indian species, Discognathus lamta ; this 
has been described under at least 20 names. It is a 
species that has a very wide geographical range, 
being found as far west as Palestine, and is liable to 
local variation.* 
On the whole, therefore, in deciding between two 
specimens, are they varieties of the same species, or 
different ? the safest rule seems to be as follows : — 
Do they differ in only one respect, and is that a 
point with regard to which the individuals of that 
particular genus are usually inclined to be variable ? In 
this case, the chances are that they belong to the same 
species, of which one or the other may be a local variety. 
Fishes that have been long kept in confinement, 
or exposed to unnatural conditions are eminently 
liable to variation. 
* Tlie reason for tlie wide range of Discognathus lamta seems to be 
that it is a species living in rapid hill streams among the rocks and 
bonlders ; mountain ranges would thus, so long as they did not rise 
above the snow level, present little or no impediment to its spreading, 
as they do to species inhabiting tlie plains only. 
