288 BULLETIN : MUSEUxM OF COMrAliATlVE ZOOLOGY. 



C. Significance of the Variations Observed. 



The objections to explaining polydactylism in the pig by the theory 

 of reversion are based on anatomical, embryological, and palaeontological 

 evidence. They have been well summed up by Gegenbaur ('80): 

 (1) tlie accessory pollex is composed of three plialanges, whereas, if 

 due to reversion, it should consist of only two ; (2) the other parts of 

 the manus show no modifications toward ancestral conditions; (3) no 

 fundament of the pollex is present at any stage in the embryo pig, nor 

 is it present as a rudiment in any artiodactyle, living or extinct. 

 Gegenbaur, accordingly, concludes that the extra digit is not produced 

 by the development of a vestige, but can be formed only from the 

 duplication of one of the normal digits. Are these objections and 

 Gegenbaur's theory supported by the cases which we have examined ? 



Pirst, as to the number of phalanges in digit i : in five of our cases 

 there was present a pollex of two phalanges. In the remaining twenty- 

 nine cases, however, there were three elements in each of the extra digits. 

 Gegenbaur is thus right in the main, but there are a few instances which 

 contradict his sweeping statement. 



As regards the modification of the other parts of the polydactyle 

 manus, Gegenbaur is again correct in his general statement. But we 

 have seen that in a limited number of cases tliere are found the identical 

 conditions which he maintains never exist. The trapezium, trapezoid, 

 and third metacarpal of the polydactyle manus resemble in structure 

 the same elements in the manus of certain fossil swine (Ancodus, Palaeo- 

 choerus). But the troclilear ridge is found at the distal articular face 

 of the metacarpals in all polydactyle conditions, although it is partly 

 or completely wanting in fossil forms. Other peculiarities of the 

 phalanges of fossil forms are not reverted to. 



The musculature also shows some interesting changes. Extensor 

 metacarpi obliquus is in many cases inserted into the metacarpal of the 

 extra digit (i) rather than into metacarpal ii. But wc know that in the 

 polydactyle manus of man tendons may shift from normal to abnormal 

 digits, although reversion plays no part in producing these abnormalities. 

 The development (1) of the extensor proprius ,pollicis et indicis (which 

 is rudimentary in the normal manus) and (2) of an independent tendon 

 from the radial side of the flexor perforans are the best evidences pre- 

 sented by the musculature that the extra digit is produced from a 

 vestige. But no great weight can be placed on the structure of the 

 muscles, as their modifications appear to be chiefly adaptive. They are 



