304 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



We have suggested the possibiUty that a factor in tlie production of 

 polydactylism in man, the cat, and the fowl may be reversion, not to a 

 Jiypothetical heptadactyle ancestor, but to the uuraodified minimus, 

 pollex or hallux of a not distantly related peutadactyle form. The re- 

 acquired structures might prove to be in their germinal characters, like 

 those of many neomorphs, so unstable as to lead to variations in tlie 

 next generation, such as polydactylous duplications. 



We have evidence to show that in man, the cat, and tlie fowl it is not 

 a definite number of extra digits, but a tendency to digital variation and 

 duplication which is inherited. In man the minimus may l)o duplicated 

 on all extremities, but to a different degree iu each case, and the varia- 

 tions may increase in succeeding generations. Thus, Fackenheim ('88) 

 cites the case of normal parents whose daughter had a rudimentary sixth 

 finger on the idnar side of each hand. Of her two sons, one liad six 

 fully developed digits on each hand, tlie other six digits on all four 

 extremities! In another family the first parent observed had six toes on 

 each foot. Of eight children three were normal, three had six toes ('\n 

 one case correlated with hare-lip), and two had six fingers ; all the extra 

 digits were of symmetrical occurrence. In the three succeeding genera- 

 tions extra digits appeared now on the feet, now on the hands, and in two 

 cases on all four extremities. In two cases also, seven toes were present 

 on one or both feet. 



In a family of cats observed by Poulton ('86) the abnormality ap- 

 peared in the third generation (number of extra digits not stated). Iu 

 the fourth generation six toes appeared on all four extremities. In the 

 fifth generation there were many individuals with seven toes on all paws, 

 and evidences of further duplication in the existence of doubled claws. 

 All gradations occurred between the extreme and normal form. This 

 condition prevailed up to the ninth generation, although in every case 

 the male parent was normal. 



Torrey (:02) describes a similar case in which the offspring of a female 

 cat with six toes on the nianus and five on the pes showed all gradations 

 between the normal and a seven-toed condition. Often in these cats the 

 pollex was abnormally long and composed of tjiree phalanges instead of 

 two. In all cases digits ii-v were apparently normal in structure. 



Bateson's breeding experiments show the same to be the case in the 

 polydactylous fowl. On crossing with normal birds all degrees of 

 variation are exhibited by the hallux, from simple elongation to 

 complete duplications and reduplications. 



These observations bring out the important fact that often no extra 



