300 BULLETIN : MUSEUM OF COMrARATIVK ZOOLOGY. 



has yet to be proved that such modifications are inlierited. Certain cases 

 of digital duplication are undoubtedly caused by the pressure of amniotic 

 threads. Such abnormalities are true malformations, and usually alFect 

 a normal, unreduced digit. An assured case is that of a duplicated 

 thumb described by Ahlfeld, in which a fold of the amnion was found 

 at birth still adherent between the duplications of the poUex. It is 

 possible that certain cases where a single functional digit is duplicated 

 are produced in a similar manner. Such examples of polydactylism, 

 however, are the exceptions ratlier than the rule, for in both mammals 

 and birds we have seen that the typical, unmodified, functional digits 

 vary but rarely. Under this class might come the cases of partial or 

 complete duplication of digits ii-iv in birds and man; of digits ii-v in 

 carnivores ; of digits in and iv in artiodactyles, and of digit in in the 

 horse. Some cases of the duplication of digits i and v in man and of 

 digits II and v in swine may also be included in the above categor}^ ; but 

 it may be that all the symmetrically placed, hereditary digital abnormali- 

 ties are produced by some internal influence emanating from the germ 

 itself. 



2. Internal Influences. 



One of the most important facts brought out by the comparative stmly 

 of polydactylism is its limitation chiefly to the variation of digits which 

 normally are either modijied, rudimentary^ or vestigial. It is natural to 

 conclude that all such variations are due to one and the same cause. 

 But on comparing the diff"erent types we find that it is only in the liorse, 

 ruminants, swine, and the pes of carnivores that extra digits arise as 

 vestigial developments ; whereas, in man, the fowl, and the manus of 

 the cat they are formed as duplications of functional digits. 



a. Reversio7i. 



The theory of reversion, first proposed by Darwin to account for poly- 

 dactylism in man, has been supported, and extended to all mammahan 

 forms, by Bardeleben ('85), Albrecht ('86), Kollman ('88), Cowper ("89), 

 and Blanc ('93). Boas ('85, '90) limits reversionary polydactylism to 

 the horse and ox. Marsh ('92) asserts that the digital variations in the 

 Equidae can be accounted for in no other way. Gegenbaur ('80, '88), 

 while strongly opposed to the theory in general, admits that it may 

 bo applicable to polydactylism in the horse. 



Reversion, as generally understood, is but heredity carried to an 

 extreme in point of time. It is the inheritance by an individual of 



