GEN. SALMO. THE COMMON TROUT. 137 



spotted with small black spots, and are generally 

 more silvery. The colours certainly accommodate 

 themselves, as Mr. Wilson remarks, to the tint of 

 the water, and to the prevailing tone of the bottom, 

 whether of rock or gravel, or softer substance ;* and 

 whatever may be the proximate cause of this, there 

 can be no doubt that it contributes to their conceal- 

 ment and consequent safety, just as we observe an 

 assimilation of colour to the places they frequent 

 so often do in the case of land animals. 



Trouts may almost be said to perform a kind of 

 local migration, for under the influence of the same 

 instinct which brings Salmon from the sea, they 

 leave the deep pools in the lower portions of the 

 river, and push upwards towards the sources in 

 search of shallow currents, which are best adapted 

 for the development of the ova. These they deposit 

 usually in the end of November. According to 

 Mr. Shaw the young of the Salmon-trout and of the 

 Common Trout are so strikingly alike, that it is ex- 

 tremely difficult to distinguish them. Trout are in 

 best condition from the end of May till near the 

 close of September ; that is to say, during the season 

 when they are best supplied with food, and when 

 alone they can obtain insect food, on which they so 

 much depend. The average weight of Trout, espe- 

 cially in the rivers of the Lowlands of Scotland, may 

 be stated to be from half a pound to three-quarters ; 

 of course they occasionally occur of more consider- 

 able dimensions. The largest found in the Tweed 

 * Ichthyology, Ency. Brit., 7th edit., p. 207. 



