AS TO THE nEEEBlTT OF ACQUIEED CONDITIONS. 459 



Fischflosse iu eine Butrachierhand mit aufgeuommen wurden. 

 Hyperdaktylie des Menschen is demnach eine besondere Form 

 des Atavismus." 



As to the mesial form, there is more difficulty in deciding. 

 According to some authorities, as, for example, Ablfeld*, 

 Polydactyly is the result of the compression of the growing 

 digits by amniotic bands, the depth and amount of the excess 

 depending upon the extent of the constriction. Two points 

 should, however, be noted in this connection : — (1) The mal- 

 formations which are accounted for by pressure of the mem- 

 branes or by constriction from bands derived from them, or from 

 other sources which will be more particularly considered when the 

 subject of peromelia is dealt with, are gradually being reduced 

 in number as our knowledge increases. Many conditions attri- 

 buted to these causes are now satisfactorily accounted for in 

 other ways. In connection with the present subject, it may be 

 said that there is little, if any, evidence to show that amniotic 

 bands have ever anything to do with the production of polydactyly. 

 (2) It is not easy to understand how a malformation caused in 

 this way could become hereditary. The heredity, if any exist, 

 must run in one of two lines — an heredity of the amniotic bands 

 causing the malformation, or an heredity of a malformation 

 first caused by a band. The first, which, by the way, has been 

 advanced, seems too improbable to require much considera- 

 tion. The coincidence by which an inflammatory band could 

 form in exactly the same positiou, and produce the same effect in 

 tlie pregnancies of several generations, is one which can scarcely 

 be admitted. And the second mode would be nothing more or 

 less than the inheritance of a mutilation. But Weismaun has 

 proved, I think it will be universally acknowledged, in his various 

 writings, that a mutilation has at least never been shown to have 

 been transmitted. Laying aside, however, these two considera- 

 tions, we have two kinds of polydactyly— the marginal, which it 

 is highly probable is always atavistic and blastogenic ; and the 

 central, which may be due to excess of material in the germ, and 

 therefore blastogenic, or, possibly, to amniotic bands, and there- 

 fore somatogenic. 



It is instructive, then, to inquire which of these is the most 

 common. According to Forster, a supernumerary little finger 



* ' Die Missbildungen des Meneclien ' (Jena, 1865), S. 43. 



