Darwinism and the evolution of man / by H.C. March.

  • March, Henry Colley.
Date:
1883
Catalogue details

Licence: Public Domain Mark

Credit: Darwinism and the evolution of man / by H.C. March. Source: Wellcome Collection.

    29/96 (page 21)
    4. Rudiments.—Organs or parts of organs that have ceased to perform any function, and especially such as, from whatever cause, have become withered, wasted, or attenuated, are called rudiments. The wing of the ostrich is a rudimentary organ. In this animal the bones of the skeleton are almost entirely destitute of the air-cells which give so much lightness to ordinary birds. The great size and strength of its legs, however, render its pace in running exceedingly swift. The changes that we see in the domestic duck, which walks more and flies less than its wild parent, have in the ostrich extended much further, and we find here that the wings are small in size and quite incapable of flight, though they possess bones which both in number and in form agree with those of the wings of other birds. We can have no hesitation in attributing the rudimentary condition of the wing of the ostrich to its disuse. Some rudiments are so shrunk and minute as hardly to be explained in this way. After an organ has ceased being used, and has become in consequence much reduced, how can it be still further lessened until the merest vestige is left, as in the apteryx, and other wingless birds ; and how can it be finally quite obliterated? It is scarcely possible that disuse can go on pro- ducing any further effect after the organ has once been rendered functionless. Some additional explanation, says Darwin, is here requisite, which I cannot give. The additional factors are probably these two :—i. Hygroscopic and thermic influences. The drying influence of the air and the astringent action of cold on disused external organs. In some observations made of the metamorphosis of lizards, it was remarked that the gills and tail-fin seemed to shrivel by a simple drying process, through the action of the air, in opposition to a true absorption by the vital processes of the animal itself. The effects of cold may be shown as follows :—Mice having become troublesome on the arctic island of St. Paul, a seal-fishing station, cats were brought to keep them in order. In three generations these cats not only became shorter in the body, and covered with a thicker fur, but they lost a greater portion of their tail. 2. The second factor is that organs in process of evolution are able to grow at the expense of those that are not required; whereby the latter must undergo a progressive diminution. It is well known that the existing horse walks on the extremity of a single gigantic toe. The foot (leg), however, has two small splint-bones, one on each side, which are rudiments of ancestral digits {fig. K.). The horse's precursor in Pleiocene times was Hipparion, an animal about the size of an ass, which had these lateral toes structurally developed, but small and functionless (fig. L.). The Meiocene horse, Anchithernim, about