PULMONARY EVOLUTION IN MAMMALIA 195 
requirements of the organism, lies at the root of the bronchial 
architecture of the extant mammalia. In place of mechanical 
modification of a preformed structure we see everywhere in the 
mammalian bronchial tree the selection of developmental paths 
from a field only limited by the extent of the pulmonary entoderm 
itself. Selection of the number, location and direction of the 
points at which epithelial proliferation is focused in bronchial 
development furnishes the archeal types from which the extant 
patterns have passed to the modern descendants. For any 
given mammalian group the force of inheritance will ordinarily 
assemble the entodermal buds progressively as they develop in 
the embryo according to a pattern, which outlines the type of. 
bronchial tree characteristic for the form in question in the nor- 
mal adult individual. 
Comparative anatomy indicates clearly that the forces which 
determined the selection of this pattern in the ancestor were en- 
vironmental and metabolic in character. A primeval ancestral 
bronchial type transmitted to the descendants can be altered and 
modified by the same morphogenetic processes determining the 
selection of new bronchial lines, as soon as the descendants 
themselves, or a large proportion of them, have begun, under 
changed conditions of environment and adaptation, to constitute 
new mammalian types requiring such respiratory modifications. 
An example of this is furnished in the rodent subfamily of the 
Hydromyinae: Xeromys, a purely terrestrial form, exhibits the 
dominant asymmetrical bronchial type, common to most mem- 
bers of the order, with the eparterial development confined to 
the right side. Its close relative, Hydromys, is strictly aquatic 
in habit, and develops a bilateral symmetrical eparterial district. 
The same condition is encountered in Myopotamus, in which a 
very extensive left eparterial bronchus appears in association 
with the equally well developed left cardiac bronchus. 
These instances of left eparterial bronchial extension in some 
of the aquatic rodents place themselves in line with the corre- 
sponding pulmonary organization found in its full development 
in the pinnipedia and some cetaceans. Like these they are 
examples of pulmonary adaptations to specific environmental 
and functional conditions. 
