Chap. V. USE AND DISUSE. 137 



The eyes of moles and of some burrowing rodents are 

 rudimentary in size, and in some cases are quite covered 

 up by skin and fur. This state of the eyes is probably 

 due to gradual reduction from disuse, but aided perhaps 

 by natural selection. In South America, a burrowing 

 rodent, the tuco-tuco, or Ctenomys, is even more subter- 

 ranean in its habits than the mole ; and I was assured 

 by a Spaniard, who had often caught them, that they 

 were frequently blind ; one which I kept alive was cer- 

 tainly in this condition, the cause, as appeared on dis- 

 section, having been inflammation of the nictitating 

 membrane. As frequent inflammation of the eyes must 

 be injurious to any animal, and as eyes are certainly 

 not indispensable to animals with subterranean habits, 

 a reduction in their size with the adhesion of the eye- 

 lids and growth of fur over them, might in such case be 

 an advantage ; and if so, natural selection would con- 

 stantly aid the effects of disuse. 



It is well known that several animals, belonging to the 

 most different classes, which inhabit the caves of Styria 

 and of Kentucky, are blind. In some of the crabs the 

 foot-stalk for the eye remains, though the eye is gone ; 

 the stand for the telescope is there, though the telescope 

 with its glasses has been lost. As it is difficult to ima- 

 gine that eyes, though useless, could be in any way 

 injurious to animals living in darkness, I attribute their 

 loss wholly to disuse. In one of the blind animals, 

 namely, the cave-rat, the eyes are of immense size ; and 

 Professor Silliman thought that it regained, after living 

 some days in the light, some slight power of vision. In 

 the same manner as in Madeira the wings of some of 

 the insects have been enlarged, and the wings of others 

 have been reduced by natural selection aided by use 

 and disuse, so in the case of the cave-rat natural selec- 

 tion seems to have struggled with the loss of light and 



