294 Dispersion of Fresh- water Fishes 



we can with some confidence look for intermediate forms where 

 the territory occupied by the one bounds that inhabited by the 

 other. In very many such cases the intermediate forms have 

 been found ; and such forms are considered as sub-species of one 

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

 as an offshoot due to the influences of different environment. 

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

 or less readily recognizable, there are varieties and variations 

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

 but still 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 will rarely fail to find some sort of 

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

 viously the reflex of differences in the environment, and the 

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

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



FIG. 188. Scartichthys enosimce Jordan and Snyder, a fish of the rockrpools of 

 the sacred island of Enoshima, Japan. Family Blenniidce. 



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 

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

 from northern waters 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. 



