4 INDIAN FRESH-WATER FISHES. 



be necessary to know in order to assign to the fish its 

 proper position in the system. 



Each family, and each genus, has certain distinc- 

 tive peculiarities which are common to all the fishes 

 in that family or genus, and at the same time there 

 are certain other points which are inclined to be 

 variable in that family, or that genus, and by the aid 

 of such peculiarities is one species to be discriminated 

 from another. 



For example, the Browns might be characterised 

 by a hooked nose, and the Joneses by a cast in the 

 eye, but to describe the difference between two 

 Joneses, say Tom and Harry, you would have to 

 notice some other detail that was variable in the 

 Jones family. 



To give a familiar expression to my argument. 

 Suppose a traveller saw a monkey for the first time, 

 and described it as " a creature with a long tail, 

 that lives in trees." Obviously such a description 

 as this would throw no light on its classification, as 

 it might equally refer to a squirrel, or to a lizard. 

 On the other hand, if he was to describe it as a 

 " quadrumanous mammal," a naturalist would know 

 at once what kind of a creature it was, even though 

 monkeys had never previously been heard of. 



For this reason it has been necessary, in describing 

 new species of fish, to use the most strictly technical 

 terms, of the exact meaning of which there can be 

 no doubt, and to avoid ordinary colloquial expressions, 

 which might bear various meanings. 



