158 Muscular Variations 
specialization and differentiation carried to the fullest possible degree— 
masking antecedent conditions, or combining them with other elements 
which make the determination of homologies a matter of difficulty. 
In the light of more extended knowledge the commonly accepted 
relative phylogenetic position of many forms requires careful revision. 
Kspecially does the view which regards the Primates as a comparatively 
late and structurally higher derivative from the mammalian stem meet 
with reversal at the hand of the morphological facts. In some respects 
man and the other Primates offer simpler morphological conditions than 
are found in other mammalian orders, and conversely many of the so- 
called “lower animals” are, in respect to certain structural specializa- 
tions, far in advance of the stage occupied by the Primate homologues. 
Deductions, to be of phylogenetic value, must be drawn with the knowl- 
edge and appreciation of two facts: 
a. The impulse to perpetuate muscular structure along primitive 
ancestral lines is a strong and influential factor in determining normal 
myological conditions for any given species, and in the production of 
muscular variants. This factor is active, notwithstanding the well 
known tendency of muscular structures to adapt themselves extensively 
by migration, concrescence, metamorphosis and fibrous regression to 
skeletal modifications and to changes in their functional application. 
In dealing with muscular homologies it is of the greatest importance 
to reckon with these changes, to differentiate between the secondary 
acquired characters and the primitive fundamental elements. Especial 
care must be exercised in assigning the correct morphological value to 
the skeletal connections, and here again the variability of the area of 
origin requires particular consideration. 
b. Under altered functional requirements rehabilitation may take 
place of muscles which practically at one period in the evolution of the 
species had become rudimentary, but which under the stimulus of a 
new physiological application, resume an active place in the organism, 
combining at times with adjacent muscles to form compounds of a 
puzzling myological character. 
Modern investigations of the muscular system should meet the fol- 
lowing demands: 
Accurate determination, for the muscle or muscular group in ques- 
tion, of the norm and of the range of variation, with full consideration 
of the nerve supply, as the decisive factor in establishing homologies. 
These data should be based on the examination of an adequate number 
of individuals of the species under consideration. 
Full and detailed comparison, on the same lines, with the correspond- 
