The Descent of Man. Part 



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

 Btrictly xTidimentary, 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. Eudimentary 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 

 i 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. 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 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 bo 

 very small,^ are diflcult 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,^* I need 

 here say no more on this head. 



Eudiments of various muscles have been observed in many 

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



^^ Some good criticisms on this Zoolog. 1852, torn, xviii. p. 13) de- 



Bubject liave been given by Messrs. scribes and figures rudiments ot 



Murie and Mivart, in 'Transact, what he calls the " muscle pedieux 



Zoolog. Soc' 1869, vol. vii. p. 92. de la main," which he says is some- 



'^^ ' Variation of Animals and times " infiniment petit." Another 



Plnnts under Domestication,' vol. ii. muscle, called " le tibial post&ieur," 



pp. 317 and 397. See also ' Origin is generally quite absent in the 



of Species,' 5th edit. p. 535. hana, but appears from time to time 



^* For instance M. Richard (^ An- in a more or less rudimeuitary coiv 



luiles des Sciences Nat.* 3rd series, dition. 



