[POLYDACTYLISM.
Supernumerary fingers and toes are eminently liable, as various authors have insisted, to be inherited. Polydactylism graduates (12/27. Vrolik has discussed this point at full length in a work published in Dutch, from which Sir J. Paget has kindly translated for me passages. See, also, Isidore Geoffroy St. Hilaire 'Hist. des Anomalies' 1832 tome 1 page 684.) by multifarious steps from a mere cutaneous appendage, not including any bone, to a double hand. But an additional digit, supported on a metacarpal bone, and furnished with all the proper muscles, nerves, and vessels, is sometimes so perfect, that it escapes detection, unless the fingers are actually counted. Occasionally there are several supernumerary digits; but usually only one, making the total number six. This one may be attached to the inner or outer margin of the hand, representing either a thumb or little finger, the latter being the more frequent. Generally, through the law of correlation, both hands and both feet are similarly affected. Dr. Burt Wilder has tabulated (12/28. 'Massachusetts Medical Society' volume 2 No. 3; and 'Proc. Boston Soc. of Nat. Hist.' volume 14 1871 page 154.) a large number of cases, and finds that supernumerary digits are more common on the hands than on the feet, and that men are affected oftener than women. Both these facts can be explained on two principles which seem generally to hold good; firstly, that of two parts, the more specialised one is the more variable, and the arm is more highly specialised than the leg; and secondly that male animals are more variable than females.
The presence of a greater number of digits than five is a great anomaly, for this number is not normally exceeded by any existing mammal, bird, or reptile. 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 through reversion. These facts are rendered, as Professor Huxley has observed, more remarkable from its being known in most cases that the affected person has not married one similarly affected. In such cases the child of the fifth generation would have only 1-32nd part of the blood of his first sedigitated ancestor. Other cases are rendered remarkable by the affection gathering force, as Dr. Struthers has shown, in each generation, though in each the affected person married one not affected; moreover, such additional digits are often amputated soon after birth, and can seldom have been strengthened by use. Dr. Struthers gives the following instance: in the first generation an additional digit appeared on one hand; in the second, on both hands; in the third, three brothers had both hands, and one of the brothers a foot affected; and in the fourth 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 the first appearance of additional digits in unaffected families are much more frequent than cases of inheritance. Many other deviations of structure, of a nature almost as anomalous as supernumerary digits, such as deficient phalanges (12/29. Dr. J.W. Ogle gives a case of the inheritance of deficient phalanges during four generations. He adds references to various recent papers on inheritance 'Brit. and For. Med.-Chirurg. Review' April 1872.), thickened joints, crooked fingers, etc., are, in like manner, strongly inherited, and are equally subject to intermission, together with reversion, though in such cases there is no reason to suppose that both parents had been similarly affected. (12/30. For these several statements see Dr. Struthers 'Edinburgh New Phil. Journal' July 1863 especially on intermissions in the line of descent. Prof. Huxley 'Lectures on our Knowledge of Organic Nature' 1863 page 97. With respect to inheritance, see Dr. Prosper Lucas 'L'Heredite Nat.' tome 1 page 325. Isid. Geoffroy 'Anom.' tome 1 page 701. Sir A.