300 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
has yet to be proved that such modifications are inherited. Certain cases 
of digital duplication are undoubtedly caused by the pressure of amniotic 
threads. Such abnormalities are true malformations, and usually affect 
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 pollex. 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 rather 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 11—-1v in birds and man; of digits 1—v in 
carnivores ; of digits m1 and Iv in artiodactyles, and of digit m1 in the 
horse. Some cases of the duplication of digits 1 and v in man and of 
digits 11 and Vv in swine may also be included in the above category ; 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 study 
of polydactylism is its limitation chiefly to the variation of digits which 
normally are either modified, rudimentary, or vestigial. It is natural to 
conclude that all such variations are due to one and the same cause. 
But on comparing the different types we find that it is only in the horse, 
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. Reversion. 
The theory of reversion, first proposed by Darwin to account for poly- 
dactylism in man, has been supported, and extended to all mammalian 
forms, by Bardeleben (85), Albrecht (86), Kollman (88), Cowper (89), 
and Blane (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 
be 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 
