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Show 136 LAWS OF VARIATION. CHAP. v. cealed, until the wind lulls and t~e sun shines ; that the proportion of wingless be~tles. IS larger on the. ex-d Dezertas than in Madeua Itself; and especially pose . . d b M the extraordinary fact, so strongly InSiste on. y r. "\\r ollaston, of the almost entire abs~nce of certain large groups Of beetles, elsewhere excessively numerou.s , a.n d hi h oups have habits of life almost necessitating w c gr "d . h d frequent flight ;-these several consi ~r~tions ave rna e e believe that the wingless condition of so many :adeira beetles is mainly due to the action of natural selection but combined probably with disuse. For during thousands of successive g~nerations e~ch i~dividual beetle which flew least, mther from Its Wings having been ever so little less perfectly 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 will oftenest have been blown to sea and thus ba ve been destroyed. The insects in Madeira which are not ground-feeders, and which as the flower-feeding coleoptera and lepidoptera must habitually use their wings to gain their subsistenc~ have as Mr. Wollaston suspects, their wings not ' ' . . . at all reduced, but even enlarged. This IS quite com-patible 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 num?er of in?ividuals were saved by successfully battling with the WI~ds, or by giving up the attempt and rarely or nev~r 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 have been better for the bad swimmers if they had not been able to swim at all and had stuck to the wreck. HAP, V. USE AND DISUSE. 137 The eyes of moles and of some burrowing rodents are rudimentary in size, and in some cases are quite covered up 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 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 indispensable to animals with subterranean habits, ~ reduction in their size with the adhesion of the eyehds and growth of fur over them, might in such case be an advantage; and if so, natural selection would constantly aid the effects of disuse. It is.well known that several animals, belonging to the most different classes, which inhabit the caves of Styria and of Kentucky, are blind. In some of the crabs the foot-stalk for the eye remains, though the eye is gone ; t~e s~and for the telescope is there, though the telescope With Its glasses has been lost. As it is difficult to ima? i~e . that eyes, though useless, could be in any way InJUriOus to animals living in darkness, I attribute their loss wholly to disuse. In one of the blind animals namely, the cave-rat, the eyes are of immense size· and Professor ~illiman. thought that it regained, after iiving some days In the hght, some slight power of vision. In the ~arne manner as in Madeira the wings of some of the. Insects have been enlarged, and the wings of others have been reduced by natural selection aided by use a.nd disuse, so in the case of the cave-rat natural selectiOn seems to have struggled with the loss of light and |
