170 EFFECTS OP USB AND DISUSE. [Chap. V. 



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 of 

 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 virreck. 



The eyes 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 per- 

 haps by natural selection. In South America, a bur- 

 rowing 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 ap- 

 peared on dissection, 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 necessary to animals having subter- 

 ranean habits, g, reduction in their size, with the adhe- 

 sion of the eyelids and growth of fur over them, might 

 in such case be an advantage; and if so, natural selec- 

 tion would aid the effects of disuse. 



It is well known that several animals, belonging to 

 the most different classes, which inhabit the caves of 

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

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



