PULMONARY EVOLUTION IN MAMMALIA 129 
tered depend upon differences in the number, mode of assembly and 
distribution of like fundamental structural units, adapted to the 
quantitative factor of a uniform and stable respiratory function, 
not upon the development of units specially modified to meet a 
wide range of physiological demands,’ varying qualitatively in 
the different types. The respiratory epithelium is the same 
wherever encountered, just as the air, with which it comes into 
contact in the exercise of its specific function, is a practically 
uniform admixture of the same chemical elements. The differ- 
ences in the morphological organization of the lung in different 
mammalian types depend solely upon two factors: 
1. The extent of the respiratory epithelial surface required to 
meet the physiological demands of the individual types. 
2. The mode of organizing the system of conducting tubes 
through which the air is supplied to the respiratory area, and 
which of course is largely dependent in turn on the surface extent 
presented by the same. The question is never what kind of 
air, but how much air does any given mammalian organization 
require to run its own specific and individual machine, and how 
can that air be delivered most promptly and efficiently to the 
same. 
Consequently, given the same ratio of respiratory exchange 
for any number of mammalia, with a diversified environment 
determining their evolution in various directions, they will de- 
velop into correspondingly varied groups, leaving the pulmon- 
ary structure untouched during these adaptations, provided 
these demand no change in the respiratory requirements of the 
organism. But as soon as environmental or functional condi- 
tions arise which alter the equilibrium of the preexisting balance 
between respiration on the one hand and all the remaining ac- 
tivities of the organism on the other, or which specifically change 
the functional aspects of respiration, they are registered in the 
resulting architectural changes of pulmonary structure. As 
above outlined the full development of the eparterial component 
in mammals coincides with great development of the skeletal 
and muscular systems and specific forms of locomotion, or with 
the mammalian adaptations to aquatic life. In over 90 per cent 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 27, NO. 2 
