250 bulletin: museum of comparative zoology. 



the abnormal digits to form systems of minor symmetry. (5) Tolydac- 

 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 conchules that the 

 abnormalities are generally constant in position, but variable in degree. 

 In reviewing the different theories advanced to account for polydactyl- 

 ism ho 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 liave 

 been proposed to account for the occurrcince of digital variation : (1) Re- 

 version, or Atavism. (2) External stimuli (pressure of amnion in xitero). 

 (3) Internal stimuli (germinal variation)/ A discussion of these theo- 

 ries Avill be more in place after we have examined for ourselves the types 

 of polydactylism occurring in the diiferent 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 palingenetic 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, Euraiuantia, and Equidae). 



