160 Museular Variations 
We are evidently dealing here with an accidental displacement of a 
portion of the otherwise normal Pectoralis major, which has, in con- 
sequence of an error in the differentiation of the muscle, resulted in the 
formation of an accessory intermediate pectoral shp. ‘This slip has 
followed the lead of the Pectoralis minor in gaining a more proximal 
insertion, while its origin shows it clearly to be the missing portion of 
the Pectoralis major. 
The case speaks for the original common derivation of both Pectoral 
muscles, but the error in differentiation is an accidental and fortuitous 
occurrence without special reversional significance. 
Progressive Variations.—A still more limited number of muscular 
abnormalities may be placed in the division of Progressive variations. 
as departures from the normal type possessing the significance of indi- 
cating evolutionary processes which will eventually lead to permanent 
pronounced structural modifications of existing muscles, or to the acqui- 
sition, as normal constituents of the organism, of new muscular integers. 
This class of variations is illustrated by the cases in which the Pero- 
neus tertius defaults, and is replaced by a short extensor of the 5th toe 
added to the Extensor Brevis Digitorum. 
Reversional Variations —In number and diversity—as well as in fre- 
quency of occurrence—the variations belonging to this subdivision far 
exceed all the others. They offer reversional indications of the stages 
through which, in the phylogenetic history of the muscular apparatus, 
the specifie myological types of to-day have obtained their individual 
structural character. 
The most complicated and specialized muscles of living forms owe 
their present high degree of development and individuality to progressive 
modifications of 2 primitive ancestral muscular system, presenting the 
typical metameric arrangement still preserved in the lowest living verte- 
brates and in the ontogeny of the higher forms. 
Closely dependent upon the further elaboration of the skeleton, and 
directly associated with the same as an integral part of the locomotory 
apparatus, the muscle-groups and individual muscles appear as deri- 
vatives from this common stem, varying in their degree of separation 
from, or combination with adjacent muscular groups, and capable of 
further numerical augmentation and specialization by cleavage and di- 
vision (Pl. II, Fig. 2.) 
In following the evolution of any given muscular group a point will 
be reached at which the definition of the special type characteristic of 
the great vertebrate classes begins. From the antecedent common ver- 
