98 SCIENCE SKETCHES. 



whose differentiation is of long standing, species, 

 therefore, which can be readily distinguished from 

 one another. When, on the other hand, we have 

 " representative species," closely related forms, 

 neither of which is found within the geographical 

 range of the other, 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 con- 

 sidered as sub-species of one species, the one 

 being regarded as the parent stock, the other 

 as an offshoot due to the influences of differ- 

 ent 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 obviously the reflex of dif- 

 ferences 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, 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, dependent on the color of the sur- 

 roundings. Fishes in large bodies of water reach 



