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 
