THE LAWS OF VARIABILITY. 51 



The insects in Madeira which are not 

 Page 109. 



ground-feeders, and which, as certain flower- 

 feeding Coleoptera and Lepidoptera, must habitually use 

 their wings to gain their subsistence, have, as Mr. Wol- 

 laston suspects, their wings not at all reduced, but even 

 enlarged. This 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 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 farther, 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 rodent 

 — the tuco-tuco, or ctenomys — is even more subterranean 

 in its habits than the mole ; and I was assured by a Span- 

 iard, who had often caught them, that they were fre- 

 quently blind. One which I kept alive was certainly in 

 this condition, the cause, as appeared on dissection, hav- 

 ing 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 ani- 

 mals 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. 



