Page:The New International Encyclopædia 1st ed. v. 06.djvu/374

* DISUSE. 320 DISUSE. its disappearanee."' An cNaiiiple he gives of tlie roiluction and loss of orpins or limbs is that of snakes, which by clianj^c of habit from orig- inally running to gliding direi-tly along the ground, have acquired greater length of body, while their legs. bi>ing thus disadvantageous to them, have disappeared by atrophy. He also instances the loss by disuse of teeth in the baleen whales and in birds, and in the ant- eater, "whose habit of not mastii'aling its food has iK-cn for a long time established and pre- served in its rac-e." He calls attention to the small reduced fore legs of the kangaroo, which "have remained thin, very small, and weak," and especially to the whale, saying: "Indeed, since the enormous length of time iluring which these animals have lived in the depths of the sea, never using their hind feet in seizing objects, their disused feet have wholly disappeared, as also their skeleton, and even the pelvis serving as their attachment," Lamarck taught that the effects of disuse are inherited, a doctrine maintained by Darwin, who remarks: "1 think there can be no doubt that use in our domestic animals strengthens and enlarges certain parts, and disuse diminishes llicm ; iviid that such modifications arc inherited." adding that in free nature "many animals have structures which can be explained by the etfects of disuse." See LsE-IxiiKRiT.xtE. Darwin also calls atteution to the absence in many male dung-beetles, including the Atcuclius or "sacred' beetle of the Egyptians, of the an- terior tarsi or feet, and this atrophy he thinks it safe to regard "as due to the elTcets of long- continued disuse." The same may be said of the reduction and loss of the fore tarsi of butter- tlies of the family X nphalid!e. For cases of the loss of eyesight, of eyes, and the optic nerves and lobes in animals living in darkness, see C.WE AXIM.VLS. Cases of the modification of animals by disuse occur in nearly every group. Among the mol- lusks it is especially noticeable in those bivalves, such as the oyster, anomia, etc., which live at- tached to other bodies. The oyster was originally equivalve and capable of freely moving, but by di.-eds, nuts, etc. ; in the larvic of gall-Hies, of ants, wasps, Ik-cs, and in the larva- of the ichneumon-flies. If, as is very clearly the case with insect.-^, we also attempt to explain the loss of limbs in the two-legged llipes. I'hirotes. or the entirely legless Aniphis- ba-na and the glass-snake (Ophiosaurusl . we shall -ec that it is evidently due to disuse and its inherited effects. Reasoning from this to the origin of the order of snakes, it iM'comes apparent that thty have diverged from the lizards, and that Lamarck was riglit when he attributed their origin to such a cause, (See Degeneration.) The majority of snakes possess no trace of vestigial legs, the exception being the boas, in whiih there are two minute hurnlike vestiges ])rnjecting from nii.l.r the scales, and repre-onl- ing the hinder pair of limbs. Disuse has also made its mark in the running birds, the emu, ostrich, and cassowary, where the wings have by disuse retrograded, and as n compensation the legs and jiarticularly the toes have become adapted for a cursorial mode of locomotion. Other examples of disu.se are the loss of the thumb in the thumb- less ring-monkey (Colobus) of .frica. and the spider-monkey (.Ateles) of Central and South America. It is to be observed in these cases of the reduc- tion and loss of digits that it is generally com- plete and thoroughgoing: there are no vestiges left as seen in the two-tood ostrich and the three- toed nioa. aptcryx. emu, and cassowary. The reduction is complete, and is probably due to the high antiquity of these birds. That this is the case is suggested by the presenc-e of the two splint-bones of the horse, while the bones of the two other toes which began to be lost at an earlier geological age have entirely lapsed. In the potto ( I'erodicticus). a lemur which has partly lost the index-finger, there is a distinct vestige left, the reduction having apparently taken place lat« in the ontogeny of this form. In the case of the loss of wings in insects, of which there are so many cases, we have all grades of reduction. Some are completely apterous, with no vestiges of wings; in others there are little flaps left. The absence of wings in all these cases is clearly the result of change in the surroundings, or mode of life, as the wing- less beetles, etc.. which live under stones, or burrow in the ground. A very interesting question here arises: Could these losses ever be restored? When we consider forms like the stick-insect, whose ancestors of the Carboniferous period had large, well-devel- oped wings: or the flea, and the Hraula, the sheep-tick (Melophagus) and the bat-tick ( Xyc- teribia I . it seems impossible that they could be restored to their ancestral winged form, even if the attempt were made to do so artificially in a laboratory of exiK'rimental evolution. Other eases are the loss outright of the collar-bone in running mammals like the horse and most carnivora. of the canines in rodents, and all the teeth in the ant-eaters. It may be considered as contrary to the course of nature to suppose that tliese structures, once degenerated and lost, could ever be restored. See Degeneh.tion. Here arises a question: Are there examples of the replacement of organs lapsed by disuse by entirely new structures? The clearest case is that of the duckbill of .iistialia. Poulton has shown that the enibrA'o duckbill has for a brief season the rudiment.s of thrM' teeth in each upper maxilla, but that these are very soon lost, wliile their site is occupied by the posterior set of the eight horny plates situ- ated in the upper and lower jaws, two upon each side of each jaw. The c:ilcificd true teeth of Ornithorhynchns became. PiMilton thinks, un- suited to the needs of the animal when it adopted a mo■ ; ■■..iitinued succession: or a constantly grow-