PULMONARY EVOLUTION IN MAMMALIA 115 
2. Assuming that the dominant type of the lung in modern 
mammalia, with elimination of its left eparterial component, re- 
ceived no compensation for this loss by augmentation in other 
directions, the primitive lung, in respect to expanse of respira- 
tory area and functional efficiency, attained a higher degree of 
development than is exhibited by the vast majority of extant 
types. 
3. As far as actual evidence is concerned this deduction from 
the tenets of the Reduction Theory is opposed both by palaeon- 
tological and comparative anatomical evidence. The line sepa- 
rating the general reptilian and mammalian organizations of 
to-day is nowhere more sharply drawn than in the distinction 
between poikilo- and homoeothermal vertebrates, which rests in 
the last analysis upon the different ratio of tissue-combustion in 
the two groups. This in turn is determined by the difference 
in the rate of the respiratory metabolism and is dependent upon 
the degree of structural development attained by the respira- 
tory organs. It is scarcely conceivable that the descendants of 
a reptilian ancestry, with relatively low pulmonary organiza- 
tion, should enter the path of mammalian evolution with such a 
superabundant respiratory endowment as to necessitate its re- 
duction in the course of their further advance. On the other 
hand the weight of the comparative anatomical data leads to 
the conclusion that the bilateral eparterial bronchial develop- 
ment is favored, if not genetically contingent upon, certain en- 
vironmental and functional factors, such as aquatic habitat and 
resultant periods of very active respiration alternating with 
suspension of the function during submersion, great bulk of the 
musculature, rapid or long continued locomotion, ete. All these 
may have played an important part in association with a higher 
pulmonary organization, evidenced by the more extensive epar- 
terial development of the cetaceans, pinnipede carnivores and 
ungulates. Were these or similar environmental influences de- 
cisive in the evolution of the ancestral mammalia, and account- 
able for their high degree of pulmonary expansion? The Reduc- 
tion Theory would demand that the early mammalia were bulky, 
aquatic forms with a pulmonary unfolding so far in excess of the 
