RUDIMENTS. 13 



l onger subject to natural selection. They often become 

 wholly suppressed. When this occurs, they are neverthe- 

 less liable to occasional reappearance through reversion a 

 circumstance well worthy of attention. 



The chief agents in causing organs to become rudiment- 

 ary seem to have been disuse at that period of life when 

 the organ is chiefly used (and this is generally during matu- 

 rity), and also inheritance at a corresponding period of life. 

 The term * 'disuse" does not relate merely to the lessened 

 action of muscles, but includes a diminished flow of blood 

 to a part or organ, from being subjected to fewer altera- 

 tions of pressure, or from becoming in any way less habitu- 

 ally active. Rudiments, however, may occur in one sex 

 of those parts which are normally present in the other sex; 

 and such rudiments, as we shall hereafter see, have often 

 originated in a way distinct from those here referred to. 

 In some cases, organs have been reduced by means of nat- 

 ural selection, from having become injurious to the species 

 under changed habits of life. The process of reduction is 

 probably often aided through the two principles of com- 

 pensation and economy of growth; but the later stages of 

 reduction, after disuse has done all that can fairly be at- 

 tributed to it, and when the saving to be effected by the 

 economy of growth would be very small,* are difficult to 

 understand. The final and complete suppression of a part, 

 already useless and much reduced in size, in which case 

 neither compensation nor economy come into play, is 

 perhaps intelligible by the aid of the hypothesis of pangene- 

 sis. But as the whole subject of rudimentary organs has 

 been discussed and illustrated in my former works, f I need 

 here say no more on this head. 



Rudiments of various muscles have been observed in 

 many parts of the human body ;J: and not a few muscles, 



* Some good criticisms on this subject have been given by Messrs. 

 Murie and Mivart. in " Transact. Zoolog. Soc," 1869, vol. vii, p. 92. 



f " Variation of Animals and Plants under Domestication," vol. ii, 

 pp. 317 and 397. See also "Origin of Species." 



\ For instance M. Richard (" Annales des Sciences Xat., 3d series, 

 Zoolog., 1852, tom. xviii, p. 13) describes and figures rudiments of 

 what he calls the "muscle pedieux de la main," which he says is 

 sometimes " infiniment petit." Another muscle, called " le tibial 

 posterieur," is generally quite absent in the hand, but appears from 

 time to time in a more or less rudimentary condition. 



