Natiiral History of the River TroiU. 169 



the gillarroo and the charr, who feed on analogous 

 substances ; and the trout, that have similar habits, 

 might be expected to resemble them. When trout 

 feed on small fish, as minnows, and on flies, they 

 have more tendency to become spotted, and are 

 generally more silvery." 



The latter remarks apply well to the Thames 

 trout, which are all black spotted and red fleshed. It 

 has been suggested that the Salmo Levenensis owes 

 its colour to feeding very much on the water-snail 

 Lymnea fluviatilis and it has been noted that in 

 rivers where that snail is prevalent the trout have 

 pink flesh. 



It Is also noted that in the swift-running streams, 

 over gravelly beds, the fish, for the most part, have 

 red spots and yellow flesh perhaps owing to the 

 greater exposure to light ; in the deeper rivers and 

 more sluggish streams trout are more black spotted. 



Sir Humphry Davy, "Salmonia," p. 67, in 

 considering the varieties of trout, says : " Fish in 

 a clear, cool river, that feed much on larvae, and 

 that swallow their hard cases, become yellower, 

 and the red spots increase so as to out-number the 

 black ones, and these qualities become fixed in 

 young fishes, and establish a particular variety." 

 But such is hardly the case. A silvery, black- 

 spotted fish, taken out of his river, or put into 

 another part of it, where red spots predominate, 



