128 GEO. S. HUNTINGTON 
locomotion, ete. The great number of the divergent type owe 
their production to differences in the form of the alimentation 
adopted by each. In the final analysis the modifications of the 
skeletal and muscular systems arose in response to the primary 
necessity of obtaining food, and the manifold morphological 
differentiations of the intestinal tract are the structural expres- 
sions of the variety encountered in the types of alimentation. 
The one relation of the organism to the exterior which remains 
fundamentally unaltered under all conditions of mammalian 
evolution is the respiratory function. The air breathed by the 
mammal, save for slight variation dependent upon altitude, 
temperature, aqueous vapor, and other extraneous admixtures, 
remains physically and chemically the same for all forms, no 
matter how widely their relations to the milieu may differ in 
other respects. 
The primitive intestinal canal of the mammal differentiates 
hence phylogenetically along two distinct lines, leading to results 
which differ correspondingly in their morphological detail and 
physiological significance. 
The strictly alimentary portion of the tube reflects in its 
structure the enormous diversity of the physiological work 
assigned to it in the various types in strict conformity with the 
correlated diversity in the character of their food. This produces 
the various types of glandular derivatives, the structural modifi- 
cations in length and caliber, the division into differentiated seg- 
ments and compartments, the provision of the valvular apparatus 
and sphincters, the variation in the supplementary structures of 
the oral cavity, tongue, lips, salivary glands and teeth, the pre- 
hensile modifications of the extremities and other parts of the 
body, ete. The mammalian organization compasses the entire 
range of the digestible material included within the environ- 
mental limits to which its members have become adapted, but 
all mammals breathe the same air. Consequently the respira- 
tory entodermal derivatives, in contrast to the alimentary modi- 
fications, present a simple and uniform structure. The mam- 
malian lung conforms to but a single basic morphological type, 
no matter to what animal it belongs. The variations encoun- 
