112 THE ORIGIN OF SPECIES 



rarely or never flying. As with mariners shipwrecked near a coast, 

 it would have been better for the gocd swimmers if they had been 

 able to swim still further, whereas it would have been better for 

 the 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 rudimen- 

 tary 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 tucotuco, or Ctenomys, is even more sub- 

 terranean 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, 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 reduction 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 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 Carniola and Ken- 

 tucky, are blind. In some of the crabs the footstalk for the eyes 

 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 may be at- 

 tributed to disuse. In one of the blind animals, namely, the cave- 

 rat (Neotoma), two of which were captured by Professor Silli- 

 man at above half a mile distance from the mouth of the cave, 

 and therefore not in the profoundest depths, the eyes were lus- 

 trous and of large size; and these animals, as I am informed by 

 Professor Silliman, after having been exposed for about a month 

 to a graduated light, acquired a dim perception of objects. 



It is difficult to imagine conditions of life more similar than 

 deep limestone caverns under a nearly similar climate; so that, in 

 accordance with the old view of the blind animals having been 

 separately created for the American and European caverns, very 

 close similarity in their organization and affinities might have been 

 expected. This is certainly not the case if we look at the two whole 

 faunas; and with respect to the insects alone, Schiodte has re- 

 marked: "We are accordingly prevented from considering the en- 

 tire phenomenon in any other light than something purely local, 



