12 



The Descent of Man. 



Part I. 



conditions which now exist. Organs in this latter state are not 

 strictly rudimentary, but they are tending in this direction. 

 Nascent organs, on the other hand, though not fully developed, 

 are of high service to their possessors, and are capable of further 

 development. Rudimentary organs are eminently variable ; and 

 this is partly intelligible, as they are useless, or nearly useless, 

 and consequently are no longer subjected to natural selection. 

 They often become wholly suppressed. When this occurs, they 

 are nevertheless liable to occasional reappearance through 

 reversion— a circumstance well worthy of attention. 



The chief agents in causing organs to become rudimentary 

 seem to have been disuse at that period of life when the organ 

 is chiefly used (and this is generally during maturity), 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 alternations of pressure, or 

 from becoming in any way less habitually active. Eudiments, 

 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 natural 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 compensa- 

 tion and economy of growth ; but the later stages of reduction, 

 after disuse has done all that can fairly be attributed to it, and 

 when the saving to be effected by the economy of growth would be 

 very small, 23 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 can come into 

 play, is perhaps intelligible by the aid of the hypothesis of 

 pangenesis. But as the whole subject of rudimentary organs 

 has been discussed and illustrated in my former works, 24 I need 

 here say no more on this head. 



Eudiments of various muscles have been observed in many 

 parts of the human body; 25 and not a few muscles, which are 



23 Some good criticisms on this 

 subject have been given by Messrs. 

 Murie and Mivart, in ' Transact. 

 Zoolog. Soc' 1869, vol. vii. p. 92. 



24 ' Variation of Animals and 

 Plants under Domestication,' vol. ii. 

 pp. 317 and 397. See also < Origin 

 of Species,' 5th edit. p. 535. 



25 For instance M. Richard ( f An- 

 nates des Sciences Nat.' 3rd series. 



Zoolog. 1852, torn, xviii. p. 13) de- 

 scribes and figures rudiments of 

 what he calls the " muscle pe'dieux 

 de la main," which he says is some- 

 times " infiniment petit." Another' 

 muscle, called " le t.'bial post<Srieur," 

 is g^nerallv quite absent in the 

 hana, but appears from time to time 

 in a more or less rudimentary oojv 

 dition. 



