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Show 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 direction. Nascent organs, on the other hand, though not fully developed, are of high service to their P?ssessors, and are capable of further development. . ~udimenta_ry organs are eminently variable; and this 1s partly mtelligible, as they are useless ?r nearly useless, and consequently are no longer subJected to natural selec·tion. They often become wholly s?ppressed. ~hen this occurs, they are nevertheless habl~ t? occ~swnal reappearance through reversion; and this 1s a circumstance 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 become 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 alterna~ions of p_ressure, ~r from becoming in any way less habitually active. Ruchments, 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 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, after disuse has done all that can fairly be attributed to it, and when tbe saving to be effected by the economy of growth would be very small,19 are difficult to understand. The final and com-t9 Some good criticisms on this subject have been giv~~ by Messrs. Mmie and Mivart, in' Transact. Zoolog. Soc.' 1869, vol. vn. p. 92. CUAP.I. RUDIMENTS. 19 plete suppression of a part, already useless and much reduced in size, in which case neither compensation nor econ?my can come into play, is perhaps intelligible by the md of the hypothesis of pangenesis, and apparently in no other way. But as the whole su~ject of rudimentary organs has been fully discussed and illustrated in my former works, 20 I need here say no more on this head. Rudiments of various muscles have been observed in ma~y parts of the human body; 21 and not a few muscles, whwh are regularly present in some of the lower animals can occ~s.ionally be detected in man in a greatly reduced conditiOn. Every one must have noticed the power . ~·hich m~ny _animals, especially horses, possess of movmg or tw1tchmg their skin; and this is effected by the panniculus carnosus. Remnants of this muscle in a_n efficie~t state are found in various parts of our bodies ; for mstance, on the forehead, by which the eyebrows are raised. The platysma myoides, which is well developed on th? neck, belongs to this system, but cannot be voluntarily brought into action. Prof. !urner, of Edinburgh, has occasionally detected, as he I~forms me, muscular fasciculi in five different situatw~ s, namely in the axillre, near the scapulre, &c., all of whwh must be referred to the system of the panniculus. He has also shewn 22 that the 1nusculus sternalis or sternalis brutorum, which is not an extension of the rectus abdominalis, but is closely allied to the panniculus, oc- 20 'Variation of Animals and Plants under Domestication,' vol. ii. PP~ 13~7 a~d 397. See ~lso 'Origin of Species,' 5th edit. p. 535. For mstance M. Rwhard (' Annales des Sciences Nat.' 3rd series Zoolog. 1852, tom. xviii. p. 13) describes and fi"'m·es rudiments of what ~~ cal~s the" ~u~le pedieux de la main," which he says is sometimes mfimment petit. ·Another muscle, called "le tibial posterieur" is generally quite ~bsent in tho hand, but appears from time to time 'in a more or less rudimentary condition. 22 Prof. W. Turner, 'Pl'Oc. Royal Soc. Edinburgh,' 1866-67, p. 65. c 2 |
