264 GENERAL BIOLOGY 



course of evolution, various different and wholly 

 unrelated forms have become adapted to live under 

 the earth itself. Omitting the Protozoa in the soil 

 of which, as yet, we know but little, and the animals 

 that merely burrow in the ground for protection, we 

 find that these various and diverse forms have all 

 become modified in much the same way. In the 

 water of subterranean caverns we usually find 

 various Crustacea (crayfish and their relatives), 



FIG. 91. A deep-sea fish (Slomiwt boa) with luminous organs (pli<t<- 

 phorc8) along the sides. (From Hickson, after Filhol.) 



salamanders, and fishes. In every case these are 

 blind, the same conditions apparently having pro- 

 duced the same consequence in all. In the absence 

 of light eyes are useless, and Nature brings about 

 their atrophy (just how, is a matter of speculation 

 upon which biologists are not agreed). On the 

 other hand, in the abysses of the sea, the weak 

 light that filters down from the surface is appar- 

 ently sufficient to make vision possible, and we do 



