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Show 294 LAWS OF VARIATION. CHAP. XXIV. point the limb was cut off, the deficient part, and no more, was exactly reproduced. Even with man, as we have seen in the twelfth chapter, when treating of polydactylism, the entire limb whilst in an embryonic state, and supernumerary digits, are occasionally, though imperfectly, reproduced after amputation. When a diseased bone has been removed, a new one sometimes "gradually assumes the regular form, and all the "atta,chments of muscles, ligaments, &c., become as complete as before." a This power of regrowth does not, however, always act perfectly: the reproduced tail of a lizard differs in the forms of the scales from the normal tail : with certain Orthopterous insects the largo hind legs are reproduced of smaller size: 4 the white cicatrice which in the higher animals unites the edges of a deep wound is not formed of perfect skin, for elastic tissue is not produced till long aftorwards.5 "The activity of the nisus "for·mativus," says Blumenbach, "is in an inverse ratio to the age of the " organised body." To this may be added that its power is greater in animals the lower they are in tho scale of organisation; and animals low in the scale correspond with tho embryos of higher animals belonging to the same class. Newport's observations 6 afford a good illustration of this fact, for he found that "myriapods, whose highest development scarcely " caiTios them beyond the larvre of perfect insects, can regenerate limbs and " antennre up to the time of their last moult;" and so can the larvre of true insects, ?ut not the matme insect. Salaman.ders correspond in development With the tadpoles Ol' larvre of the tailless Batmchians, and both possess to a large extent the power of regrowth; but not so the mature tailless Batracbians. Absorption ~ften plays an important part in the repairs of injuries. When a bone IS broken, and does not unite, the ends are absorbed and roundo~, s~ that a false joint is formed; or if the ends unite, but overla the proJectmg parts are removed.7 But absorption comes into action ap, v· h k d · < , <s n:c ow :·e~ar s, mmg the normal growth of bones; parts' which are sohd ~mmg yo~th become hollowed out for the medullary tissue as the bone mcreases rn size. I?- trying to understand the many well-adapted cases of regrowth when a1ded by absorption, we should remember that most parts of the organisation, even whilst retaining the same form undergo cons~ant renewal; so that a part which was not renewed would natmally be liable to complete absortion. Some cases, usually classed under the so-called nisus jormativus at first appear to come under a distinct head; for not only are old st;uctures :eproduce~, but structures which appear new are formed. Thus, after mflarnn:atwn "false membranes," furnished with blood-vessels, lymphatics, and nerve~, are developed; or a fretus escapes from the Fallopian tubes, ~nd falls rn~o the ab~ome~, "nature pours out a quantity of plastic "lymph, which,rorms Itself rnt? orga~sed membrane, richly supplied with blood-vessels, and the fretus IS nounshed for a time. In certain cases of 3 Carpenter's 'Principles of Comp. Physiology,' 1854, p. 479. 4 Charlesworth's' 1\fag. of Nat. Hist.,' vnl. i., 1837, p. 145. 5 Paget, ' Lectures on Surgical Pathology,' vol. i. p. 239. 6 Quoted by Carpenter, 'Comp. Phys., p. 479. 7 Paget, ' Lectures,' &c., p. 257. CHAP. XXIV. EFFECTS OF USE AND DISUSE. 295 hydrocephalus the open and dangerous spaces in the skull are filled up with new bonos, which interlock by perfect serrated sutures.8 But most physiologists, especially on the Continent, have now given up the belief in plastic lymph or blastema, and Virchow 9 maintains that every structure, new or old, is formed by the proliferation of pre-existing cells. On this view false membranes, like cancerous or other tumours, are merely abnormal developments of normal growths; and we can thus understand how it is that they resemble adjoining structures; for instance, that " false membrane in the "serous cavities acquires a covering of epithelium exactly like that which " covers the original serous membrane; adhesions of the iris may be~ome "black apparently from the production of pigment-cells like those of the "uvea." 10 No doubt the power of reparation, though not always quite perfect, is an admirable provision, ready for various emergencies, even for those which occur only at long intervals of time.U Yet this power is not more wonderful than the growth and development of every single creature, more especially of those which are propagated by fissiparous generation. This subject has been here noticed, because we may infer that, when any part or organ is either greatly increased in size or wholly suppressed through variation and continued selection, the co-ordinating power of the organisation will continually tend to bring all the parts again into harmony with each other. On the Effects of the Increased Use and Disuse of Organs. It is notorious, and we shall immediately adduce proofs, that increased use or action strengthens muscles, glands, sense-organs, &c. ; and that disuse, on the other hand, weakens them. I have not met with any clear explanation of thjs fact in works on Physiology. Mr. Herbert Spencer 12 maintains that when muscles are much used, or when intermittent pressure is applied to the epidermis, an excess of nutritive matter exudes from the vessels, and that this gives additional development to the adjoining parts. That an increased flow of blood towards an organ leads to its greater development is probable, if not certain. Mr. Paget 13 thus accounts for the long, thick, and dark-coloured hair which occasionally grows, even in young children, near oldstanding inflamed surfaces or fractured bones. vVhen Hunter s These cases are given by Blumenbach iu his 'Essay on Generation,' pp. 52, 5!-. 9 'Cellular Pathology,' trans. by Dr. Chance, 1R60, pp. 27, 441. 10 Paget, 'Lectures on Pathology,' vol. i., 1853, p. 357. 11 Paget, idem, p. 150. 12 ' The Principles of Biology,' vol. ii., 1866, chap. 3-5. 13 'Lectures on Pathology,' 1853, vol. i. p. 71. |