SPECIES OF SALMONIDAE 183 



close agreement with the views of Agassiz, 

 published half-a-century earlier. 



Now in order to determine the question as 

 to whether a certain form is entitled to specific, 

 or merely to varietal rank, it is first necessary 

 to define with exactitude what it is that consti- 

 tutes a species ; and it is very doubtful whether 

 a better or more authoritative definition can be 

 found than that adopted by Dr. Day. 1 



' I shall consider species among the true 

 Salmons to be an assemblage of individuals 

 which agree together in their structure, and 

 in the development of the sexes, but differ 

 in some structural character from all other 

 fishes/ 



Taking, then, this definition with its weighty 

 authority as our test, let us proceed to enquire 

 in how far the so-called separate species Salmo 

 ferox fulfils its requirements. 



The structural characters which have been 

 chiefly relied on as indicating specific differences 

 in the varying forms of our Salmonidae are 

 chiefly as follows. The number of vertebrae ; 

 now of these, according to Dr. Giinther, the 

 number in the common or brook trout ranges 

 from 57 to 60, and in Salmo ferox from 56 

 to 57 ; but Day records instances in undoubted 



1 British and Irish Fishes, Vol. II. p. 55. 



