246 bulletin: museum of comparative zoology. 



Tliroiigh the great kindness of Dr. W. McM. Wooilworth, Keeper of 

 the Museum of Comparative Zoology 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, tlicrefore, 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 resume 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 Avas carried on at the Zoological Laboratory of Harvard 

 University, and to Prof. E. L. jMark 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). lie 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 tlian on the feet by suggesting that the haml is 

 more specialized than the foot, and therefore more likely to vary. For 

 the same reason polydactylism is less common in women, tlie male 

 showing always greater ditferentiation, and therefore a greater tendency 

 to variation. Darwin at first assumed polydactylism to be reversion to 

 a more primitive ancestral condition ; but this assumption was later 

 withdrawn. 



Gegenbaur ('80) criticises the theory which regards polydactylism as 

 atavistic. His arguments are : (1) tliat other parts of the manus or pes 

 shew no correlated modifications; (2) tliat man normally possesses five 

 digits, the typical number for vertebrates, and that the supernumerary 



