18 
THE DESCENT OP MAN. 
Part I. 
which now exist. Organs in this latter state are not 
strictly rudimentary, hut 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. Rudimentary 
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 
fiom 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 saviwr 
to be effected by the economy of growth would be very 
small , 19 are difficult to understand. The final and com- 
10 Some good criticisms on this subject have been given by Messrs. 
Muric and Mivnrt, in ‘ Transact. Zoolog. Soc.’ 1869, vol. vii. p. 92. 
