PRENTISS: POLYDACTYLISM IN MAN AND DOMESTIC ANIMALS. 299 
Rosenberg (’73) searched for such vestiges, but without success. 
Ewart (’94), in tracing out the skeletal development of the limbs of the 
horse, found cartilaginous nodules articulating in an imperfect manner 
with the distal epiphyses of metacarpals 1 and Iv. The vestige 
attached to digit 11 was the larger, and in some instances showed evidence 
of division into two or three parts, which Ewart takes to be the funda- 
ments of as many phalanges. 
This is an interesting and important discovery, since, if digit 11 is better 
developed than tv in the normal embryo, we have a good explanation 
for the fact that in polydactyle horses it is the second digit which is 
of most frequent occurrence. Dissection of the manus of a foetus 35 cm. 
long enabled me to confirm Ewart’s work. There is thus conclusive 
evidence that in the horse extra digits are frequently of vestigial origin. 
The digital abnormalities of the Equidae can therefore be divided into 
two distinct classes : 
(1) Vestigial cases, in which the extra digits are developed from 
rudiments normally present in the manus of equine embryos and 
extinct ancestors. 
(2) Teratological cases, which are malformations usually due to the 
partial or complete duplication of the functional digit (a1). 
VIII. Theories of Polydactylism. 
The occurrence of polydactylism has been attributed to two proximate 
causes: (1) External influences, (2) Internal influences. 
1. ExTerRNAL INFLUENCES. 
The supporters of this theory (Ahlfeld, ’85-86, and Zander, ’91) would 
explain all cases of digital variation as due to the pressure of amniotic 
threads iz utero. This view accounts satisfactorily for the variation in 
degree of digital duplications, but utterly fails to explain their fixed 
position with reference to certain digits, and cannot apply to the 
development of digital vestiges. Pressure from an amniotic thread 
would naturally affect any finger or toe, whereas we know that poly- 
dactylism in mammals is practically limited to the first or fifth digit, is 
often bilaterally symmetrical in its occurrence, and may affect both 
manus and pes in the same individual. The abnormalities are also 
strongly inherited, and the amniotic theory, if correct, would necessitate 
admitting the inheritance of acquired characters. Although the duplica- 
tion of organs has been artificially produced by Dareste (91) and others, it 
