THE DESCENT OR ORIQIN OF MAN 29 



of such slight service to their present possessors that wo 

 can hardly suppose that they were developed under the 

 conditions which now exist. 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 possessors, and 

 are capable of further development. Eudimentary 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 be- 

 come wholly suppressed. When this occurs, they are never- 

 theless liable to occasional reappearance through reversion 

 — a circumstance well worthy of attention. 



The chief agents in causing organs to become rudimen- 

 tary 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, but includes a diminished flow of blood to a part 

 or organ, from being subjected to fewer alternations of press- 

 ure, or from becoming in any way less habitually active. 

 Eudiments, however, may occur in one sex of those parts 

 which are normally present in the other sex ; and such rudi- 

 ments, 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 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 econ- 

 omy of growth; but the later stages of reduction, after dis- 

 use has done all that can fairly be attributed to it, and when 

 iho c?Ying to fee effected by the economy of growth would 

 be very smalV are difficult to understand. The final and 

 complete supprefeJon of a part, already useless and much 

 reduced in size, in which case neither compensation nor 



'5 Some good criticisms on'^this subject have been giyen by Messrs. Murle 

 and Mlvart, in "Transact. Zoolo|, See. " 1869, vol. vii. p. 92. 



