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. 
