88 AMERICAN FISHES. 



I mention this, merely by way of suggestion, as offering an interesting 

 subject of investigation for naturalists. 



Sir Humphrey Davy, in his Salmonia, rather leans to the idea that 

 the Gillaroo is a distinct species, though he leaves it uncertain whether 

 it may not be a permanent variety ; his principal argument being 

 this, that he has caught small fish, not longer than the finger, with a 

 fly, " which had as perfect a hard stomach as the larger ones, with the 

 coats as thick in proportion, and the same shells within." 



In external appearance, the Gillaroo is said to differ from the com- 

 mon Trout " very little, except that they have more red spots, and a 

 yellow or golden-colored belly and fins, and are generally a broader 

 and thicker fish." Again, Sir Humphrey admits that " in a clear and 

 cool river, fish that feed much on larvae, and swallow the hard cases, 

 become yellower, and the red spots increase so as to outnumber the 

 black ones ; and these qualities become fixed in the young fishes, and 

 establish a particular variety." 



This would seem, in plain English, to describe the existence of a 

 fish in the direct process of change, from the ordinary form of the 

 Trout to the Gillaroo, the feeding on the larvae of winged insects, in 

 their hard stony cases, being, as it were, a first step toward becoming 

 shell-fish eaters, and the effect being indicated in the gradual change 

 of color, though the causes have not been as yet sufficiently powerful 

 to produce the induration of the stomach. 



In America, likewise, it has been attempted to draw a distinction ; 

 and Dr. DeKay, a very accomplished and able icthyologist, although 

 perhaps — with all deference be it spoken — rather too much of an in- 

 door naturalist, and too much inclined to admit hearsay evidence, has 

 designated a species as Salmo Erythrogaster, the Red-bellied Tr >ut ; 

 which I confess I do not believe to be even a permanent variety, but 

 merely a brilliant specimen of the common Brook Trout, in its highest 

 ■ season, taken, probably, from some very bright and sunny water. In 

 this view I am fully sustained by Professor Agassiz, who has made 

 some very curious experiments, with regard to the colors of fishes, of 

 the Saknonidm especially ; and who has ascertained, beyond a doubt, 

 riot only that the Trout of different neighboring waters are affected by 

 the color and quality of the water, but that the Trout of the same rivet 

 vary in color accordingly as they haunt the shady or the sunny side of 



