84 SOME CURIOUS FACTS CONNECTED WITH 



than in the eyes of other Vertebrata. Degeneration, then, such as 

 makes the eye incapable of seeing, has not taken place ; neverthe- 

 less, the eye of the mole is reduced to total inefficiency. The 

 blindness of the mole is the result of complete degetieratioji of the 

 optic nerve, so that if images could be formed in the eye itself, 

 they could never be transmitted to the animal's consciousness. 

 I7i the embryo of the ffiole and without exception, both eyes are 

 originally connected with the brain by well-developed optic nerves. 

 This may indeed be regarded as a conclusive proof that the blind 

 mole is descended from progenitors that could see, and that the 

 total blindness of the animal has been caused by the directly 

 injurious effects of darkness on the optic nerve." 



Another case of blindness arising from living in darkness, is 

 that of a parasitic crab, Pinfwtheres, which, in its adult state, lives in 

 the " water lungs" of Holothurians. The animal has well-developed 

 eyes in its free-swimming zooea stage, and even when it enters its 

 Holothurian host, preserves these eyes, but as they grow, they 

 gradually become blind ; the brow grows forward over the eyes, 

 and finally covers them so completely, that in the oldest individuals 

 not the slightest trace of them is to be seen through the thick 

 skin ; while at the same time, the eyes seem to undergo a more or 

 less extensive retrogressive metamorphosis. 



There are numberless other instances of blindness in animals 

 from disuse, which I have no time to mention. I will only allude 

 to the curious fact that whilst some fishes living at great depths 

 are totally blind, others have immensely developed eyes. Also in 

 all the species of the cave beetle, Thachcerites, the females only 

 are blind, while the males have well- developed eyes, yet both sexes 

 live together in absolute darkness. 



These cases appear to present difficulties, yet they seem to me 

 not very difficult to account for. Unless the beetles were specially 

 created in the total darkness of the caves and the fishes speciaky 

 created for the abysses of the ocean, they must each have had a 

 tendency to greater development of the eyes, as they receded from 

 the light. The female beetle would find her food at the bottom of 

 the cave, and would soon lose the use of her eyes, but the males 

 woulduse their eyes so long as there was any light at all to guide 

 them, and the retrograde development in their case would probably 



