PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 249 



cate the descent of tlie horse from comparatively recent polydactyle 

 ancestry." 



Blanc ('93) recognizes three distinct classes of polydactylism : (1) Ata- 

 vistic, or cases where ancestral digits reappear ; (2) Teratological, or 

 cases in which either normal digits or atavistic supernumerary ones are 

 duplicated; (3) Heterogenic, or cases belonging to neither (1) nor (2). 



(1) Atavistic polydactylism. Bardeleben's theory is accepted without 

 reservation. Atavism is regarded by Blanc not as the neo-generation of 

 an ancestral digit, but merely as the development of rudiments normally 

 present in the embryo. From an examination of digital abnormalities in 

 mono-, di-, tetra-, and penta-dactylous animals he deduces the follow- 

 ing general principles : (a) the more simple the extremity, the more 

 varied and the more divergent from the normal are the forms of Polydac- 

 tyly. (6) In all species the thoracic limb presents ancestral digits more 

 frequently than the pelvic does ; this leads to the conclusion that the 

 manus has become simplified later than the pes. (c) In man the post- 

 minimus appears more frequently than the prae-pollex or prae-hallux ; 

 the reverse is true for other animals. 



(2) Teratological Polydactylism. The proximate cause of these abnor- 

 malities Blanc regards as obscure, but he favors Albrecht's ('86) view of 

 reversion to the pterygian fin rays of selachians ; the single digit of the 

 higher animals represents two of these rays fused. 



(3) Heterogenic polydactylism. This consists usually of the intercala- 

 tion of extra digits, and the producing cause is unknown. 



If Albrecht's view is accepted, Blanc proposes the following classifica- 

 tion of polydactylism : 



1. Atavistic polydactylism. 



a. Eeversion to the pentadactyle or mammalian type. 



b. Reversion to the heptadactyle or reptilian type. 



c. Reversion to forms possessing a double series of phalanges or to 



the selachian type. 



2, Heterogenic polydactylism. 



The supernumerary digits are monstrosities. 

 Bateson ('94) studied polydactylism in the cat especially, but cites and 

 fijiures a lar^e number of digital variations in the other domestic animals 

 and in man. His conclusions are : (1) Polydactylism occurs much more 

 frequently in certain species than in others. (2) Particular forms of 

 digital variation are peculiar to particular animals. (3) The abnormal- 

 ity usually occurs symmetrically placed on both sides of the body, and 

 often on both fore and hind extremities. (4) There is a tendency for 



