196 THE ORIGIN OF SPECIES 



individuals were saved by successfully battling with the 

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

 fi3'ing. 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 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 perhaps by 

 natural selection. In South America, a burrowing ro- 

 dent, the tucutucu, 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 alivj was certainly 

 in this condition, the cause, as appeared 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 subterranean habits, a redaction in their 

 size, with the adhesion of the eyelids 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 

 Carniola 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 imagine that eyes, though useless, could be in any 

 way injurious to animals living in darkness, their loss 



