ON CLASSIFICATION. 11 



The next division is that of the Genera. 



A genus may be defined, roughly, as a group of 

 fishes, which closely resemble one another in all im- 

 portant details of their structure, and differ from all 

 other genera in one or more essential points ; it may 

 contain only one species, or it may contain a very 

 great number ; the genus Barbus, for example, in 

 Gimther's Catalogue, numbers over 160 species. 



It is, as may be supposed, a difficult matter to 

 decide what points are of sufficient importance to con- 

 stitute a genus, and as almost every writer on the 

 subject considers himself qualified to form his own 

 opinion, and to bestow names of his own choosing, 

 the result is a most embarrassing confusion. 



It must be borne in mind that the members of each 

 separate family have a tendency to vary in certain 

 respects, and to remain constant in certain other 

 points. 



The Siluridse, for instance, are eminently variable 

 as to their teeth, the Cyprinidse, on the other hand, 

 vary very slightly in this respect. 



Hence it is easy to understand that a slight differ- 

 ence in the number of their teeth between two 

 Cyprinoid fishes would suffice to indicate that they 

 belong to different genera, whereas a much 

 greater difference might exist between the teeth of 

 two Siluroid fishes who belong to one and the same 

 genus ; in their case it might only show that they 

 were distinct species. 



It is useful to remember that the first scientific 



