Variation of animals and plants under domestication, v.2

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12 INIIERITAN CE. CnAP. XII. finitely diversified characters, by which t~1e domestic r~ces of animals and plants are distinguished, nothmg. need Le saicl ; for the _very existence of persistent races proclaims the power of inheritance. A few special cases, however, deserve .so:ne consideration. It. might have been anticipated, that dev1atwns from the law of symmetry would not have been inherited. But ~nderso~ 23 states that a rabLit produced in a litter a young ammal havmg o~1y one ear· and from this animal a Lreecl was formecl wlnch steadily ~roduced one-eared rabbits. He also mentions a bit~h, with a sino·le leg deficient, and she produced several pup1nes with the sa~e deficiency. From Hofacker's account 24 it appears that a one-horned stag was seen in 1781 in a forest in Germany, in 1788 two, and afterwards, from year to year, many were observed with only one horn on the right side of the head. .A cow l9st a horn by suppuration,25 and she produced three calves which had on the same side of the heacl, insteacl of a horn, a small bony lump attached merely to the skin ; but we here approach the doubtful subject of inhcritecl mutilations. A man who is left-handed, and a shell in which the spire turns in the wrong direction, are 'departures from the normal though a symmetrical condition, and they are well known to be inherited. Polydactylism.-Supcrnumcrary fingers and toes arc eminently liable, as various authors have insisted, to transmission, but they arc noticed. hero chiefly on account of their occasional regrowth after amputation. Poly­dactylism graduates 26 by multifarious steps from a mere cutanoou,c; ap­pendage, not including any bono, to a double hand. But an aduitionnl digit, supported on a metacarpal bono, and furni shc<.l with all the proper muscles, nerves, and ves els, is sometimes so perfect, that it escapes detec­tion, unless the fingers arc actually counted. Occasionally there nrc several supernumerary digits; but usually only one, making the total number six. This one may Toprcscnt either a thumb or :fingcT, being attached to the inucT or outer margin of the band. Gcncmlly, tlu·ouglt the law of correlation, both hands and foot arc similarly affected. I b::wo tabulated the cases recorded in various works or privately communicated ~3 ' Recreations in Agricultme aml Nat. Hi~t.,' vol. i. p. G8. . ~4 ' Ueber die l<:igcnschaften,' &c., 1828, s. 107. 2 " Bronn's 'Gcschicht.c dcr Natur,' band ii. s. 132. 26 Vrolik has discussed this point at fulllcn o-th in a work publish erl in Dutch, from w·~ iclt 1\lr. l>ngct ba bnuly tmus­btcd for me pas~agcs. See, also, hidore G~;offroy St. lliLLire· · 'Ili::~ t. ucs Ano­mnlie;;,' 1832, tom. i. p. GS-1. CnAr. XII. INTIERITANCE. 13 to me, of forty-six persons with extra digits on one or both hands and feet; if in each case all fom extremities had been similarly affected, the table would have shown a total of ninety-two hands and ninety-two feet each with six digits. As it is, seventy-three hands and seventy-five feet were thus affected. This proves, in contradiction to the result arrived at by DT. Struthers,Zl that the hands are not more frequently affected than the feet. The presence of more than :five digits is a great anomaly, for this num-ber is not ,normally exceeded by any mammal, bil'd, or existing reptile.28 Nevertheless, supernumerary digits are strongly inherited; they have been transmitted through five generations; and in some cases, after disappearing for one, two, or even three generations, have reappeared tlU"ongh reversion. These facts arc rendered, as Professor Huxley has observed, more remarkable from its being lrnown in most cases that the affected person had not married one similarly affected. In ·such cases the child of the fifth generation would have only l-32nd part of the blood of his fu·st sedigitated ancestor. Other cases are rendered remarkable by tho affection gathering force, as Dr. Struthers has shown, in each generation, though in each the affected person had married one not affected; moreover such additional digits arc often amputated soon after birth, and can seldom have boon strengthened by usc. Dr. Struthers gives the following instance : in the fu·st generation an additional digit appeared on one hand; in the second, on both hands; in the third, tbJ:ec brothers had both hands, and one of the brothers a foot affected ; and in the fomth generation all four limbs were affected. Yet we must :not over-estimate the force of inheritance. Dr. Struthers asserts that cases of non-inheritance and of tlie fu·st appearance of additional digits in unaffected families are much more frequent than cases of inheritance. Many other deviations of structure, of a natme almost as anomalous as supernumerary digits, such as deficient phalanges, thickened joints, crooked fingers, &c., are in Wre manner strongly inherited, and arc equally subject to intermission with reversion, though in such cases there is no reason to suppose that both parents had been similarly affected.29 27 'Edinburgh New Phil. Joumal,' J uly, 18G3. 28 Some great anatomists, as Cuvier anu l\1eckel, believe thttt tLe tubercle on one siue of tl.Jc hinder foot of the tailless Batrachians represents a si.'l.th di git. Certainly, when the hinder foot of a toad, as soon as it first sprouts from the tadpole, is dissected, the par­tially ossified cartilage of tllis tubercle resembles under the microscope, in a remarkable manner, a digit. But tho l1ighcst authority on such subjects, Gcgenbaur (Untersuchung. zm Vl.:'rg­l eich. anat. dcr Wirbeltl.ticre : Carpus et Tarsus, 186-±, s. G3 ), concludes that this resembbnce is not real, only super­ficial. 29 For these several statements, see Dr. Struthers, in work cited, espe­cially on intermissions in the line of descent. Prof. Huxley, 'Lectmes on our Knowledge of Organic Natme,' 1863, p. 97. With respect to inheritance, see Dr. Prosper Lucas,' L 'Hcredite Nat.,' tom. i. p. 325. IsiQ.. Geoffroy, 'Anom.,' • tom. i. p. 701. Sir A. Carlisle, in' Phil. Transact.,' 1814, p. 94. A. Walker, on 'Intermn.rriage,' 1838, p. 140, gives a case of five generations; as does Mr. Sedgwick, in' Brit. and Foreio-n Medico­Chi. rurg. Rcvi0w,' April, 18G3, p. 4G2.