12 THE DESCKNT OP MAN. 



ist. 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 pos- 

 sessors, 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 sup- 

 pressed. When this occurs, they are nevertheless liable to oc- 

 casional 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, hut in- 

 cludes 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 selec- 

 tion, 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 compensation 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,^ are difli- 

 cult 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 intelli- 

 gible 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. 



Rudiments of various muscles have been observed in many 

 parts of the human body;"° and not a few muscles, which are regu- 



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

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



'^ 'Variation of Animals and Plants under Domestication,' vol. li. 

 pp. 317 ajid 397. See also 'Orig-in of Species,' 6th edit. p. 535. 



20 For instance M. Richard ('Annales des Sciences Nat.' 3rd series, 

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



