THE PROTEUS. THE HOLE. 79 



but certain observations, which I have made on a family of 

 Proteus that I have kept for four years, incontrovertibly prove 

 that this creature is highly sensitive to diffused daylight. As 

 this contains no heat-rays, the eye of the Proteus can receive 

 no impression but that of light. Now it is impossible to 

 suppose that the eyes are now first developing in an originally 

 blind Amphibian which, like the Proteus, lives in total dark- 

 ness ; for even if such an organ could originate under such 

 circumstances it could never become permanent in the struggle 

 for existence, because it could never be of any real use in that 

 struggle. The contrary hypothesis, on the other hand, that 

 the rudimentary eyes of the Proteus are a degenerate form of 

 the more highly developed eyes of its progenitors, seems per- 

 fectly natural when we remember that all the other amphi- 

 bians have highly developed eyes, and that these, when they 

 come to the light from time to' time, use them to very good 

 purpose. 



The Mole offers another familiar and even better example. 

 This animal, whose peculiar habits are known to everyone, 

 has true eye?, from which none of the essential parts of the 

 eyes of the Yertebrata are absent, although these parts are all 

 of the simplest, almost of embryonic struct ure. 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 very small number of minute and little altered embryonic 

 cells ; the retina, in the same way, is much simpler than in the 

 eyes of other Vertebra ta. 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 in the mole is the result solely of complete de- 

 generation of the optic nerve, so that the images which are 

 probably formed in the eye itself can never be transmitted to 

 the animal's consciousness. Occasionally, however, the mole 

 even can see a little, for it has been found that both optic 

 nerves are not always degenerate 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 



