i8o BRITISH FRESH-WATER FISHES 



own opinions in competition with his. I have bestowed 

 a great deal of attention upon the habits of Salmonid<£ ; but 

 I am neither anatomist nor physiologist. Upon the question, 

 therefore, of what degree and constancy of variation in 

 structure should be recognised as constituting a distinct 

 species, my opinion would be worthless. But, having observed 

 how completely the offspring of some of Dr. Giinther's species, 

 such as the Loch Leven trout [Salmo levenensis)^ lose their 

 hereditary characteristics in adapting themselves to novel 

 environment ; how quickly the ordinary brook trout {Salmo 

 fario) acquires the appearance of the Loch Leven race when 

 submitted to similar conditions of food and habitat ; lastly, 

 how imperceptible are the gradations between the different 

 so-called species — I incline to the opinion of Fatio, Day, 

 and other ichthyologists, which accounts for the so-called 

 specific differences between the various kinds of sea, lake, 

 and river trout as the transient effects of food, climate, 

 and local environment. 



In effect, I have taken a cowardly middle course, which will 

 meet with the approval of neither party, by adopting Dr. 

 Giinther's subdivision of the genus Salmo into two groups — 

 Salmones and Salvelini^ salmon and char — and designating as 

 species the leading types of each. I feel reassured in doing so 

 by the fact that Dr. Giinther's classification is admittedly 

 tentative, was formulated nearly forty years ago, and that some 

 of his minor specific differences can scarcely be said to have 

 stood the test of subsequent observation. 



In one respect all the genus Salmo, as well as some species 

 of other genera of Salmonid^e, possess in common a remarkable 

 and highly suggestive characteristic. Greatly as the adults of one 

 species may differ in appearance from those of another, yet 

 hardly more so than individuals of the same species differ among 

 themselves, they all pass through a stage when not only all 

 members of the same species are alike, but it is scarcely possible 

 to distinguish one species from another. Yearly salmon and 



