198 GEO. S. HUNTINGTON 
here a second factor presently to be considered is chiefly 
responsible. 
The greatly increased eparterial development of the typical 
avian lung depends on the high rate of tissue-combustion, as 
evidenced by the high bodily temperature, and on the enor- 
mous relative development of the specialized pectoral muscula- 
ture with its great and long continued activity during flight. In 
spite of the fact that the weight and bulk of the whole body has 
been reduced as far as possible by special adaptations to this 
purpose, centred chiefly in the appendages of the respiratory 
tract, the remaining factor is correlated with a great cephalo- 
ventral extension of the lung and the establishment of the type 
of pulmonary organization characteristic for the class. 
3. Changes in the general environment, which are associated: 
with changes in the structure of the lung, may lead to the replace- 
ment of a slow regular respiratory rhythm by a type in which 
periods of exceedingly active respiration alternate with more or 
less prolonged intermissions during which respiratory exchange is 
at a stand still. This condition is the result of the adaptation 
of the mammalian organism in a greater or lesser degree to the 
aquatic habitat. Shorter or longer periods of submergence, 
during which respiration is suspended, are succeeded by inter- 
vals of active surface breathing. The reaction of this environ- 
mental factor on intrapulmonary organization is seen in the 
high degree of bilateral eparterial development in the pinnipede 
carnivores, sirenia and typical cetacea. 
The question of the total expenditure of respiratory energy 
within a given period of time becomes important in its effects 
on pulmonary structural modifications of evolutionary character. 
In the mammal the interdependence of the biochemical factors 
of respiration and lung-structure require analysis primarily in re- 
spect to the rapidity with which the gas-exchange is effected. 
The more extensively organized mammalian lungs indicate either 
a high degree of tissue-combustion at a fairly constant rate (Ungu- 
late type), or a very rapid intermittent metabolism called for at 
definite periods with intervals during which respiration is sus- 
pended (aquatic adaptations). 
