246 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
Through the great kindness of Dr. W. McM. Woodworth, Keeper of 
the Museum of Comparative Zodlogy at Harvard College, a valuable 
collection of polydactyle specimens was placed at my disposal. ‘The 
investigation represented by this paper was undertaken with the view 
to obtaining, from a study of these abnormalities, some clue as to the 
causes leading to their occurrence. 
In order to understand the phenomena of polydactylism, and to make 
it possible to draw some general conclusions, a comparative study of 
such abnormal structures is necessary. It has, therefore, been considered 
worth while to collate from the literature brief descriptions of poly- 
dactylism in those forms of which we were unable to obtain suitable 
material. In reviewing the literature, however, a résumé is given of 
only those papers which draw important and general conclusions. 
Works concerned chiefly with descriptions of polydactylism in individual 
animals are treated of in the separate accounts of digital, variations in 
man and the different domestic animals here referred to. 
My research was carried on at the Zodlogical Laboratory of Harvard 
University, and to Prof. K. L. Mark are due my sincerest thanks for 
both the laboratory privileges I enjoyed, and his own kind direction 
and most valuable criticism. To Dr. W. E. Castle I am also indebted 
for important criticisms and revision of proof. 
I. Historical Survey. 
Allusions to polydactylism are to be met with as far back as the time 
of Pliny. The first investigator who attempted to collect scientific data 
on the subject was Struthers (63). He tabulated digital abnormalities 
in man, and proved that they were strongly inherited. 
Darwin (’76) accounts for the fact that supernumerary digits are more 
numerous on the hands than on the feet by suggesting that the hand is 
more specialized than the foot, and therefore more likely to vary. For 
the same reason polydactylism is less common in women, the male 
showing always greater differentiation, and therefore a greater tendency 
to variation. Darwin at first assumed polydactylism to be reversion to 
& more primitive ancestral condition; but this assumption was later 
withdrawn. 
Gegenbaur (’80) criticises the theory which regards polydactylism as 
atavistic. His arguments are: (1). that other parts of the manus or pes 
shcw no correlated modifications ; (2) that man normally possesses five 
digits, the typical number for vertebrates, and that the supernumerary 
