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. 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 
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 wo 
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 saving 
to be effected by the economy of growth w^ould 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 4 Transact. Zoolog. Soc/ 1869, vol. vii. p. 92. 
