but usually with scanty markings. Soft, dull mud, more or less barren of 

 vegetable or stationary animal life, and free from stones and sand, is a char- 

 acteristic fresh-water product. Such fish as the American Hornpout (Ami- 

 urus catus, etc.) haunt this simple element, and have a correspondingly plain 

 and simple obliterative equipment. So do the fish-like young of many frogs 

 and toads and salamanders^the "tadpoles," in other words. But, as in 

 almost all other cases, there are gradations from this type of coloration into 

 others. The common American Brook Trout (Salvelinus jontinalis) varies 

 much in coloration, having even, as I have already mentioned, a 'chamele- 

 onic' power of almost instantaneously changing — ^lightening or darkening — 

 its general tint. The dullest-colored trout from a dark, muddy pond or brook 

 is only a few grades above the hornpout or the poUiwog in -fairness of tint 

 and elaborateness of pattern; while a light and bright one from a clear, shallow 

 stream is one of the most exquisitely and 'specially' marked and colored 

 of fishes. In the aspect of this fair trout's common background there is a 

 characteristic which we have not yet considered, and which in its full display 

 is peculiar to shallow running water. That is, the system of moving light- 

 and-shadow patterns on the bottom, made by sun-rays variously refracted 

 from the agitations of the surface. Everybody is familiar with these beauti- 

 ful water-patterns, that glide and flicker over the beds of brooks in sunlight. 

 They are of many forms, as the surface-agitations are of many sorts; never- 

 theless one can distinguish a few predominant types among them. The sim- 

 plest consists of more or less nearly parallel and widely separated undulant 

 bright lines; the most complicated is a maze of twisting, branching, tremulous 

 bright bands encircling darker spaces — plaided, sometimes, like the scales 

 of a fish,— or like the giraffe's pattern. These pretty effects are produced of 

 course both by wind- and current-ripples; but it is the cwrrewZ-ripples that 

 make the greatest variety of patterns. Notable among these variations are 

 the whirlpool shadows. They are cleanly and perfectly circular dark spots, 

 each surrounded by a bright rim of sunlight. In all brooks whose passage 

 is in any way obstructed at the surface, little whirlpools are engendered here 



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