130 EFFECTS OF USE AND DISUSE. 



wingless beetles is larger on the exposed Desertas than in 

 Maderia itself; and especially the extraordinary fact, so 

 strongly insisted on by Mr. Wollaston, that certain large 

 groups of beetles, elsewhere excessively numerous, which 

 absolutely require the use of their wings, are here almost 

 entirely absent. These several considerations make me be- 

 lieve that the wingless condition of so many Maderia 

 beetles is mainly due to the action of natural selection, 

 combined probably with disuse. For during many succes- 

 sive generations each individual beetle which flew least, 

 either from its wings having been ever so little less per- 

 fectly developed or from indolent habit, will have had the 

 best chance of surviving from not being blown out to sea; 

 and, on the other hand, those beetles which most readily 

 took to flight would ofteuest have been blown to sea, and 

 thus destroyed. 



The insects in Maderia which are not ground-feeders, 

 and which, as certain flower-feeding coleoptera and 

 lepidoptera, must habitually use their wings to gain their 

 subsistence, have, as Mr. Wollaston suspects, their wings 

 not at all reduced, but even enlarged. This is quite com- 

 patable 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 liave 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 

 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 Spaniard 

 who had often caught them, that they were frequently 

 Dlind. One which I kept alive was certainly in this con- 

 dition, the cause, as appeared on dissection, having been 



