170 EFFECTS OF USE AND DISUSE. [Chap. V. 



is quite compatible with the action of natural selection. 

 For when a new insect first arrived on the island, the 

 tendency of natural selection to enlarge or to reduce the 

 wings, would depend on whether a greater number o] 

 individuals were saved by successfully battling with the 

 winds, or by giving up the attempt and rarely or never 

 flying. As with mariners shipwrecked near a coast, it 

 would have been better for the good swimmers if they 

 had been able to swim still further, whereas it would 

 have been better for the bad swimmers if they had not 

 been able to swim at all and had stuck to the wreck. 



The eves of moles and of some burrowing rodents are 

 rudimentary in size , and in some cases are quite 

 covered 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 subterranean 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 certainly in this condition, the cause, as 

 appeared on dissection, Laving been inflammation of the 

 nictitating membrane. As frequent inflammation of the 

 eyes must be injurious to any animal, and as eyes are 

 certainly not necessary to animals having subterranean 

 habits, a reduction in their size, with the adhesion of 

 the evelids and growth of fur over them, might in such 

 case be an advantage ; and if so, natural selection would 

 aid the effects of disuse. 



It is well known that several animals, belonging to 

 the most different classes, which inhabit the caves of 



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

 crabs the foot- stalk for the eye remains, though the eve 



