250 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the abnormal digits to form systems of minor symmetry. (5) Polydac- 
tylism is due to variation, and not to reversion. 
Wilson (’96) gives an account of five cases in man where polydactyl- 
ism was transmitted through several generations, and concludes that the 
abnormalities are generally constant in position, but variable in degree. 
In reviewing the different theories advanced to account for polydactyl- 
ism he rejects that of reversion and Bardeleben’s prae-pollex theory on 
grounds similar to those put forward by Gegenbaur (80, ’88) and Zander 
(91), and holds that germinal variation is the proximate cause. 
If we summarize the conclusions of the various investigators whose 
work we have briefly reviewed, it appears that three explanations have 
been proposed to account for the occurrence of digital variation: (1) Re- 
version, or Atavism. (2) External stimuli (pressure of amnion zn wtero). 
(3) Internal stimuli (germinal variation). A discussion of these theo- 
ries will be more in place after we have examined for ourselves the types 
of polydactylism occurring in the different domestic animals. In pro- 
ceeding with this examination we must keep these three theories clearly 
in mind. If we are. warranted in rejecting Bardeleben’s prae-pollex 
theory, the possession of six digits by any domestic animal must be ac- 
counted for on grounds other than reversionary. And only in animals 
normally possessing fewer than five digits may we look for atavism to 
restore, either partially or completely, the typical number of digits ; 
even in these cases the supernumerary parts may be produced by the 
duplication of one or more of the normal digits. Throughout the fol- 
lowing pages, therefore, we shall endeavor to determine as definitely as 
possible the respective parts which these supposed causes play in pro- 
ducing polydactylous abnormalities. 
_ The special point which we have to determine is whether the extra 
digits which appear in polydactylism are of palingenetic or neogenetic 
origin, — whether they are returns to old structures, or represent new 
variations. The term reversion has been loosely used to designate the 
general phenomenon of heredity. To avoid confusion I shall limit its 
meaning to the abnormal inheritance of palingenetie characters, while 
heredity will be used in the broader sense. Beginning with the typi- 
cal pentadactyle extremity characteristic of man and the Carnivora, we 
shall take up in turn those forms in which the number of functional digits 
has been reduced (fowl, swine, Ruminantia, and Equidae). 
