18 THE DESCENT OF MAN. [Part L 



useless, arid consequently are no longer subjected to nat- 

 ural selection. They often become wholly suppressed. 

 When this occurs, they are nevertheless liable to occa- 

 sional reappearance through reversion ; and this is a cir- 

 cumstance well worthy of attention. 



Disuse at that period of life, when an organ is chiefly 

 used, and this is generally during maturity, together w T itb 

 inheritance at a corresponding period of life, seem to have 

 been the chief agents in causing organs to become rudi- 

 mentary. 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 parts normally present in the other 

 sex ; and such rudiments, as we shall hereafter see, have 

 often originated in a distinct manner. In some cases or- 

 gans 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 compensation and 

 economy of growth; but the later stages of reduction, 

 alter 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 ve^y small, 19 are difficult to understand. 

 The final and complete sivppression of a part, already use- 

 less and much reduced in size, in which case neither com- 

 pensation nor economy can come into play, is perhaps in- 

 telligible by the aid of the hypothesis of pangenesis, and 

 apparently in no other way. But as the whole subject of 

 rudimentary organs has been fully discussed and illustrated 

 in my former works, 20 1 need here say no more on this head. 



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

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



»o < Variation of Animals and Plants under Domestication,' vol. ii. pp. 

 817 and 397. See also c Origin of Species,' 5th edit. p. 535. 



