EXTRA DIGITS AND DIGITAL REDUCTIONS 339 



Fig. 2. A series showing the normal and polydactyl structure of the skeletal elements in 

 the pes of the dog. A, normal pes with the rudimentary metatarsal bone of digit I. ; B, a 

 polydactyl pes, the hallux represented by two phalanges and the distal end of the metatarsal 

 bone ; C, polydactyl pes with hallux (I.) completely developed. 



circumstantial evidence, but direct proof that the extremities of the 

 ungulate have been derived by evolution from a five-toed type. This 

 is an important point, but one about which investigators have been at 

 variance. Bardeleben, Kollman, Marsh, Blanc and others recognize all 

 cases of polydactylism as due to reversion. Gegenbaur warns us against 

 such general conclusions, but admits that the extra digits sometimes 

 found on the extremities of the horse are developed from the digital 

 rudiments. Weismann, Bateson and Wilson ascribe all such abnor- 

 malities to germinal variation. But germinal variation may affect the 

 rudimentary as well as the functional digits ; if tbrough such variation 

 the supposed rudiment of a thumb develops into a digit with two 

 pbalanges, germinal variation and reversion are one and the same thing. 

 To attempt to reconcile the conflicting statements of various in- 

 vestigators the writer has made a comparative study of polydactylism 

 in mammals normally possessing less than five toes. It was found that 

 in the majority of cases the extra digits are developed from the so-called 

 digital rudiments. This is most frequently observed in the pes of the 



Fig. 3. A series to show the reversion of the pig's manus to the pentadactyl condition. A, 

 Cfirpals and metacarpals of the fossil Ancodus; B, of a polydactyl pig; C, of a normal pig. 

 J.- V., first to fifth digits; trz., trapezium, the carpal element of the pollex. 



