DARWINISM AND EVOLUTION. 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 



