289 ANIMAL LIFE 
hand of man has carried the golden trout to other little 
mountain torrents, where it thrives as well as in the one 
where its peculiarities were first acquired. 
Other cases of this nature are found among the blind 
fishes of the caves in different parts of the world (Fig. 172). 
In general, caves are 
formed by the ero- 
sion or wearing of 
underground rivers. 
These streams are 
usually clear andcold, 
and when they issue 
to the surface those 
fishes that like cold 
and shaded waters 
are likely to enter 
them. But to have 
eyes in absolute dark- 
Fia. 172.—Fishes showing stages in the loss of eyes yess, in which no use 
and color. A, Dismal Swamp fish (Chologaster de of th 
avitus), ancestor of the blind fish; B, Agassiz’s can be made 0 em, 
cave fish (Chologaster agassizi); C, cave blind jig a disadvantage in 
fish (Typhlichthys subterraneus). the struggle for life. 
Hence the eyed species die or withdraw, while those in which 
the eye grows less from generation to generation, until its 
function is finally lost, are the ones which survive. By such 
processes the blind fishes in the limestone caves of Ken- 
tucky, Indiana, Tennessee, and Missouri have been formed. 
rather when specimens showing intergradation of characters are known, 
the word sub-species is used. The word variety has much the same 
meaning when used for a subdivision of a species, but it is a term 
defined with less exactness. Thus the common fox (Vulpes pennsyl- 
vanicus) is a distinct species, being separate from the arctic fox or the 
gray fox or the fox of Europe. The cross fox (Vulpes pennsylvanicus 
decussatus) is called a sub-species, as is the silver fox (Vulpes pennsyl- 
vanicus argentatus), because these intergrade perfectly with the common 
red fox. 
