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. 



* The reason for the wide range of Discognathus lamta seems to be 

 that it is a species living in rapid hill streams among the rocks and 

 boulders ; 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 the plains only. 



