Geo. S. Huntington | 161 
tebrate stem main branches are derived representing the complete mus- 
cular group or the individual muscle in e. g. Reptiles, Birds and Mam- 
mals (Pl, Il, Fig. 2), to confine the consideration, for the sake of sim- 
plicity, to the higher vertebrate types. Take, for example, the common 
mammalian myological stem. It contains primarily elements possessing 
a potential evolutionary force, capable of progressive development. lt 
can, by differentiation, cleavage and subdivision, acquire greater com- 
plexity, and the resulting new muscular organs can become highly spe- 
cialized, changing their skeletal relations by migration, acquiring new 
functional applications and entering in varying degrees into combination 
with adjacent muscles. 
On the other hand the common mammalian muscular prototype car- 
ries within it the possibility of regression and metamorphosis into 
fibrous and ligamentous tissue, or of complete default, according to the 
conditions surrounding and influencing its development in the specific 
types. New lines of adaptation to the evolutionary demands of the 
specialized mammalian orders are thus derived from the common ante- 
cedent stem of the complete mammalian muscle, which latter includes 
potentially the development of all the diverse forms assumed by the 
muscle in the various mammalian orders, genera and species. ‘Thus the 
same muscle in Man, in a Carnivore, and in a Rodent, may pursue widely 
divergent developmental paths. It may practically retain the primitive 
arrangement in one of these forms, while in another the origin and 
insertion shift by migration, or in the third the organ changes its original 
functional character and becomes partially converted into fibrous tissue 
by metamorphosis and degeneration. Yet inherently in all these diverse 
types the muscle corresponds to the fundamental mammalian class-sten 
from which the special differentiations are derived by adaptation to 
skeletal conditions and functional requirements. 
Within the limit of a single mammalian order a muscle or muscular 
group may present a number of congruent structural characters which 
lead back to the type common to the entire order. From this common 
ancestral form of the muscle the adaptations characterizing the genera 
and species of the order are derived. 
If, for example, we consider any given muscular group in reference 
to its origin from the common mammalian myological stem along the 
line of Primate adaptation, the type thus presented must include poten- 
tially the capacity of developing into the diverse forms which the muscle 
assumes in the individual families and species composing the order. 
This primate ancestral type of any muscle or muscular group may be 
reconstructed from the data furnished by the various forms assumed in 
