288 BULLETIN: MUSEUM OF COMPARATIVE 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) the accessory pollex is composed of three phalanges, 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 ? 
First, as to the number of phalanges in digit 1: 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 there 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 trochlear 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 (1) rather than into metacarpal 1. But we 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 
