204 The Troiits of America 



dark bodies assume a lighter shade, the anterior 

 streak of dirty white on their belly fins becomes 

 of a richer cream, and the wormlike markings 

 on the back change from blackish to dark olive 

 clearly and strikingly defined. These changes 

 in coloration are certainly involuntary and are a 

 provision of nature to protect them from their 

 enemies of the air and water. 



Again, we find that the difference in size and 

 in the proportions of the body in trout of the 

 same age, present further difiiculties in identi- 

 fication of species. The head especially is sub- 

 ject to great alteration as the fish grows to 

 maturity; the fins, according to Gunther, of 

 those inhabiting rapid streams, as well as still 

 waters, show considerable variations in form 

 and length ; those of the fish in rapid water 

 being in almost constant motion causes the 

 wearing off of the delicate extremities, hence 

 the fin-rays are comparatively shorter, stouter, 

 and of a more rounded form. Moreover, one 

 and the same individual may pass a part of its 

 life in a lake, and enter a river at certain periods, 

 thus changing the form of its fins, as well as gen- 

 eral coloration, almost periodically. 



Thus it will be seen that exact classification in 



