18 THE DESCENT OF MAN. Part I. 



which now exist. Organs in this latter state are not 

 strictly rudimentary, but they are tending in this direc- 

 tion. Nascent organs, on the other hand, though not 

 fully developed, are of high service to their possessors, 

 and are capable of further development. Kudimentary 

 organs are eminently variable; and this is partly in- 

 telligible, as they are useless or nearly useless, and 

 consequently are no longer subjected to natural selec- 

 tion. They often become wholly suppressed. When 

 this occurs, they are nevertheless liable to occasional 

 reappearance through reversion ; and this is a circum- 

 stance well worthy of attention. 



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

 used, and this is generally during maturity, together 

 with inheritance at a corresponding period of life, seem 

 to have been the chief agents in causing organs to be- 

 come rudimentary. 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. Rudi- 

 ments, 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 organs have been reduced by 

 means of natural selection, from having become inju- 

 rious 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 savins: 

 to be effected by the economy of growth would be very 

 small, 19 are difficult to understand. The final and com- 



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

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



