EXTRA DIGITS AND DIGITAL REDUCTIONS 337 



elements are often distinct from each other, muscle tendons and nerves 

 are bifid, and in many cases the bones of the extra digit are more or less 

 closely united to those of a normal toe. The question next arises as to 

 whether these digital bifurcations are due to external influences or to 

 internal variations of the germ plasm. Ahlfeld has observed that 

 digital duplications may be caused ' in utero ' by pressure from the 

 thread-like outgrowths of the amnion. He attempted to make this 

 explanation cover all cases of polydactvlism, but there are several 

 serious objections. In the first place, the extra digits generally occur 

 on both hands or on both feet, often on all four extremities (Fig. 1, 

 A—D). The middle digits, moreover, arc not generally affected, but 

 the duplication is chiefly of the first and fifth. Finally, and most 

 important, the abnormality is strongly inherited and may increase in 

 degree during successive generations. Thus Fackenheim cites the case 

 of a woman born of normal parents. She had the little finger dupli- 

 cated on each hand. Of two sons, one inherited the mother's extra 

 fingers and the other had besides extra small-toes on both feet. Of 

 eight children, three were normal, three had six toes and two had six 

 fingers on both right and left extremities. In three succeeding genera- 

 tions the abnormality appeared, now on the hands, now on the feet, 

 and in two cases on all four extremities; in two cases seven toes were 

 present on both feet. 



Similar observations have been made by Poulton and Torrey in 

 families of cats. It is evident that extra digits produced by the chance 

 pressure of amniotic threads would not be inherited, and that such 

 chance pressure would certainly affect now one digit, now another; 

 whereas, we have seen that the first and fifth digits are chiefly affected. 

 Of twelve cases studied by the writer all were of the latter type. 



It will be observed in Fig. 1, A-D, which represent the extremities 

 of one child, that the fifth digit is affected differently in each case. In 

 fact, it has been pointed out that occasionally no extra digit may be 

 produced, that the first or fifth digit may simply be abnormally large. 

 These facts, together with the frequent inheritance of the extra digits, 

 show that we have to do here with variations of the germplasm. The 

 first digit of man has been modified, and the fifth slightly reduced. 

 Variation most often affects organs whose structure has been recently 

 changed, and the variation or duplication of these digits might be nat- 

 urally expected. 



We are warranted, then, in assuming that the abnormal occurrence 

 of six or seven digits on the five-toed extremity is not due to reversion. 

 They are rather duplications of the normal digits, produced either by 

 externa] influences or. more frequently, by germinal variation. 



As the five-toed extremity is the primitive type of mammalian foot 

 it is but natural to conclude that the appendage with less than five toes 

 has lost some of the original number of evolutionary changes. The 



