294 



Dispersion of Fresh-water Fishes 



we can with some confidence look foi intermediate forms where 

 the territory occupied by the one bounds that inhabited by the 

 otlier. In very many such cases the intermediate forms have 

 1ieen found; and sucli forms are considered as sub-species of one 

 species, tlie one being regarded as the parent stock, the other 

 as an offshoot due to tlie influences of different environment. 

 Then, besides tliese "species" and "sub-species," groups more 

 or k'ss readily recognizable, there arc varieties and variations 

 of every grade, often too ill-defined to receive any sort of name, 

 f)ut stih not without significance to the student of the origin of 

 species. Comparing a dozen fresh specimens of almost any 

 kind of fish from any body of water with an equal number 

 from somewhere else, one Avill rarely fail to find some sort of 

 diiferences, — in size, in form, in color. These differences are ob- 

 viously the reflex of diiferences in the environment, and the 

 collector of fishes seldom fafls to recognize them as such ; often 

 it is not difficult to refer the effect to the conditions. Thus 



Fig. ISS. — Scartirhthys cnosimce Jordan and Snyder, a fish of the rock-pools of 

 tlie sacred island of Enoshima, Japan. Family BlenniidcT. 



fishes from grassy bottoms are darker than those taken from 

 over sand, and those from a bottom of muck are darker still, 

 the shade of color being, in some way not well understood, de- 

 pendent on the color of the surroundings. Fishes in large bodies 

 of water reach a larger size than the same species in smaller 

 streams or ponds. Fishes from foul or sediment-laden waters 

 are paler in color and slenderer in form than those from waters 

 wliich are clear and pure. Again, it is often true that specimens 

 from northern Avaters are less slender in body than those from 

 farther south; and so on. Other things being equal, the more 

 remote the localities from each other, the greater are these dif- 

 ferences. 



