552 



COMPARATIVE PHYSIOLOaY. 



forms the tapetum. It is most pronounced in the carnivora 

 and gives the glare to their eyes as well seen in the cat tribe at 

 night. It has been supposed to act as a reflector and thus 

 assist in vision in the same way as a pair of carriage lamps 

 light up the roadway. 



Evolntion. — From the above brief accoimt of the eye in dif- 

 ferent grades of animals, it will ap- 

 pear that its modifications answer to 

 differences in the environment. 



Adaptation is evident. Darwin 

 believes this to have been effected 

 partly by natural selection — i. e., the 

 survival of the animal in which the 

 form of eye appeared best adapted to 

 its needs — and partly by the use or 

 disuse of certain parts. 



The latter is illustrated by the 

 blind fishes, insects, etc., of certain 

 caves, as those of Kentucky ; and it 

 is of extreme interest to note that 

 various grades of transition toward 

 complete blindness ' are observable, 

 Sf4ifSnTiS,geft%^';! according to the degree of darkness 

 . er of accommodation, and ex- in which the animal lives, whether 



treme mobility of the ins. ■, ^^ ..i • ,i •■ 



wholly withm the cave or where 

 there is still some light. A crab has been found with the eye- 

 stalk still present, but the eye itself atrophied. Again, ani- 

 mals that burrow seem to be in process of losing their eyes, 

 through inflammation from obvious causes ; and some of them, 

 as the moles, have the eye still existing, though well-nigh or 

 wholly covered with skin. Internal parasites are often with- 

 out eyes. It is not difficult to understand how one bird of prey, 

 with eyes superior to those of its fellows, would gain supremacy, 

 and, in periods of scarcity, survive and leave offspring when 

 others would perish. 



It is, of course, impossible to trace each step by which the 

 vertebrate eye has been developed from more rudimentary 

 forms, though the data for such an attempt have greatly accu- 

 mulated within the last few years ; and it is not to be forgotten 

 that even the vertebrate eye has many imperfections, so that 

 no doctrine of complete adaptation, according to the argument 

 from design as usually understood, can apply. 



Pre. 399.— Eye of nocturnal bird 

 of prey (after WiederBheim). 

 Co, cornea; L, lens; St, retina; 

 P, pecten; No, optic nerve; 3c, 

 ossification of sclerotic coat; 



