PULMONARY EVOLUTION IN MAMMALIA 125 
In their application to the problem here under consideration 
these conclusions weigh heavily in favor of regarding eparterial 
pulmonary development not as the primitive generalized mam- 
malian type, destined to undergo reduction in the majority of 
the forms during further evolution, but as a condition secondarily 
acquired under the stimulus of increased functional demand, or 
through the opportunity offered by mutant variation to limited 
mammalian groups. 
4. The bilateral hyparterial bronchial type appears normally 
in very few living mammalia. 
The scattered and more or less incomplete records and obser- 
vations published on the lungs of the cetacea led Weber (36) to 
ascribe, at least tentatively, to Balaena mysticetus and B. anti- 
podum (possibly also to Orcella fluminalis) the possession of the 
hyparterial bronchial organization. It is, however, extremely 
improbable that this assumption would be sustained if the 
actual facts were thoroughly understood and correctly inter- 
preted. There remains little doubt that all cetacea have a bi- 
lateral bronchial tree either with the right eparterial bronchus 
tracheal, the left bronchial in derivation, as in the Camelidae 
and Giraffa, or, conforming to the prevalent artiodactyl type, 
with the highly developed eparterial element present only in the 
right lung and derived from the trachea. 
Among the terrestrial mammalia the bilateral hyparterial 
bronchial tree is normally encountered in a number of species 
belonging to the rodent group of the Hystricomorphs and in the 
mustelid carnivore Taxidea americana. / 
Both forms find within their respective orders fairly close 
taxonomic relatives carrying the dominant mammalian asym- 
metrical bronchial type with right sided eparterial development. 
Both agree with their nearest zooloogical relatives in respect to 
body-build, weight, habitat, mode and speed of locomotion, 
hibernation, ete., all extrinsic factors which may exercise an 
influence on the range of pulmonary. development. Likewise in 
food habits, dentition and structure of the alimentary canal they 
conform to the rodent and mustelid carnivore type. The only 
morphological character which differentiates them sharply 
