1 1 EFFECTS OF USE AND DISUSE. [Chap. V 



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 bur- 

 rowing 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 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 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 Ken- 

 tucky, 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 may be attributed 

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

 (Neotoma), two of which were captured by Professor Silliman at 

 above half a mile distance from the mouth of the cave, and there- 

 fore not in the profoundest depths, the eyes were lustrous and ol 

 large size ; and these animals, as I am informed by Professor Silli- 

 man, 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 organisation 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, Schibdte has 

 remarked, " We are accordingly prevented from considering the 

 entire phenomenon in any other light than something purely local, 

 and the similarity which is exhibited in a few forms between the 

 Mammoth cave (in Kentucky) and the caves in Carniola, otherwise 

 than as a very plain expression of that analogy which subsists 



