PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 249 
cate the descent of the 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) Atavistec polydactylism. Bardeleben’s theory is accepted without 
reservation. Atavism is regarded by Blane 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. (%) Inall 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 prac-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, Blane proposes the following classifica- 
tion of polydactylism : 
1. Atavistec polydactylism. 
a. Reversion 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 
figures a large 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 
