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Show 124 LAWS OF VARIATION. [OHAP. V. In some cases we might easily put down to ?isuse modifications of structure which are wholly, or mainly, due to natural selection. Mr. Wollaston has discovered the remarkable fact that 200 beetles, out of the 550 species inhabitinO' Madeira, are so far deficient in wings that they can~ot fly; and that of the twenty-nine endemic genera, no less than twenty-three genera have all their species in this condition! Several facts, namely, that beetles in many parts of the world are very_ frequently blown to sea and p~rish ; that the beetles in Madeira, as observed by Mr. Wollaston, lie much concealed, until the wind lulls and the sun shines ; that the proportion of wingless beetles is larger on the exposed Dezertas than-in M adeira .itself; and especially the extraordinary fact, so strongly insisted on by Mr. Wollaston, of the almost entire absence- of certain large groups of beetles, elsewhere excessively numerous, and ·which groups have habits of life almost necessitating frequent flight ;-~hese several considerations have made me believe that the wingless condition of so many Madeira beetles is mainly due to the action of naturar selection, but combined probably with disuse. For during thousands of successive generations each individual beetle which :flew least, either 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 have 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 tJ:eir subsistence, have, as Mr. Wollaston suspects, their wings not at al~ reduced, ?ut even enlarged .. This is quite compatible w1th 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 OnAP. V·] USE AND DISUSE. h' 125 s Ipwrecked near a coast it 1 the good swimmers if th wou d have been better for further, whereas it 'woulde~ had bb en able to swim still swimmers if they had not b ave bl een b~tter for the bad stuck to the wreck. een a e to SWim at all and had ~he eyes of moles and of some b . rudimentary in size and . urrowing rodents are up b Y ski·n and fur. ' This Isnt astoem oef tcla ses a r~ qui' te covered to gradual reduction from d. Ie eye~ IS probably due natural selection. In South Asus~~ but aided ~erhaps by the tuco-tuco or Cteno . menca, a burrowing rodent its habits tha~ the mole ~ys, Jr even more subterranean i~ who had often cau ht 'thn was as~ured by a Spaniard blind; one which Igke t ef, that they were frequently dition, the cause as a p a Ive wa~ certainly in this coninflammation of the ni~¥-~a~~d on dissection, having been inflammation of the eyes ~u~~\ :rr:m;nb:·ane. As frequent and as eyes are certain] not. d ~ lllJUnous to any animal, subterranean habits a [ed ;? 1~pe~sa?le .to animals with ~esion of the eyelid~ and guc I~f I~ t/eu· size with the adIn such case be an advanta:~': a~o . ur over them, might would constantly aid the ~ 't df,dl[ so, natural selection It . 1 ettec s o Isuse IS we 1 known that se · 1 · · Inost different classes ;h. hm.a h anbi.mals, belonging to the and of Kentuck ~ Ic. In a It the caves of Styria the foot-stalk for ~e eree bhn~. In smne of the crabs the stand for the teles y re:nmns, though the eye is gone . with its glasses has be~ifo~~ ti_Are.' t?o~gh the telescop~ that eyes, though useless co~ld s It ;s dtfficult to imagine to a~imals living in dark~ess I ~e.bn any :vay injurious to disuse. In one of the bl' d a. ri ute thmr loss wholly rat, the eyes are of immen In. animals, namely, the cave-t~ ought that it regained s:f~Ize i. a?d Professor Silliman hg~t, some slight power ~f vf: lVI~ some days in the as In Madeira the win s of Ion. n .the same manner enlarged, and the wings some of the Insects have been natural selection aidedgb of others have been reduced by the cave-rat natural l [.use and disuse, so in the case of the loss of light ands: eh Ion .seems to have struggled with bh~reas with all the ~th!:inh:bitsef t~e ~ze of the ~yes; y Itself seems to have done its wo~k.s o t e caves, dJsuse |