Cha?. XII. INHERITANCE. 457 



merely to the skin ; but we here encroach on the subject of 

 inher'.ted mutilations. A man who is left-handed, and a 

 shell in which the spire turns in the wrong directions, are 

 departures from the normal asymmetrical condition, and they 

 are well-known to be inherited. 



PolydactyUsm.—Su-pernVimeraYj fingers and toes are eminently 

 liable, as various authors have insisted, to be inherited. Poly- 

 dactylism graduates '■''' by mu^ltifarious steps from a mere cutaneous 

 appendage, not including any bone, to a double hand. But an ad- 

 ditional 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. Gene- 

 rally, through the law of correlation, both hands and both feet are 

 similarly affected. Dr. Bm-t Wilder has talmlated ^"^ 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 nimiber is not normally exceeded by any existing 

 mammal, bird, or reptile. Nevertheless, supernumerary digits aie 

 strongly inlierited ; they have been transmitted through five genera- 

 tions ; 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 afi'ected. In such cases the child of the fifth 

 generation would have only l-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 

 afiected ; moreover, such additional digits are often amputated soon 

 after birth, and can seldom have been strengthened by use. Dr. 



-'' Vrolik has discussed this point p. 684. 

 at full length in a work published "* ' Massachusetts Medical Society,' 



in Dutch, iVom which Sir J. Piiget vol. ii. No. 3 ; and ' Proc. Boston 



has kindly transhited for me passages. Soc. of Nat. Hist.,' vol. siv., 1871, p. 



See, also, 'Isidore GeoftVoy St. Hilaire's 154. 

 ♦Hist, des Anomalies.' 1832, tom. i. 



