THE PINEAL SYSTEM OF AMPHIBIA 239 



to diffused light," and he remarked that " as this light contains no heat 

 rays the eye of the Proteus can receive no impression but that of light." 



Semper also writes of the mole, this animal " has true eyes from which 

 none of the essential parts of the eyes of vertebrates are absent, although 

 these parts are all of the simplest embryonic structure. The whole 

 eye is very small, deeply imbedded in muscles, and quite covered by the 

 skin, so that it is quite invisible externally. The lens consists of a number 

 of minute and little altered embryonic cells. The retina, in the same way, 

 is much simpler than in the eyes of other vertebrates. True degeneration, 

 then, such as makes the eye incapable of seeing, has not taken place ; 

 nevertheless the eye of the mole is reduced to almost total inefficiency, 

 even when by chance it has an opportunity for using it. This almost 

 total blindness of the mole is the result of complete degeneration 

 of the optic nerve, so that the images which are probably formed 

 in the eye itself can never be transmitted to the animal's conscious- 

 ness. Occasionally the mole can see a little, for it has been found 

 that both optic nerves are not always degenerated in the same individual, 

 so that one eye may remain in communication with the brain, while 

 the other has no connection with it. In the embryo of the mole, and 

 without exception, both eyes are originally connected with the brain by 

 well-developed optic nerves, and so theoretically (are capable of becoming) 

 efficient. This may be regarded as a perfectly conclusive proof that the 

 blind mole is descended from progenitors that could see." 



