196 GEO. S. HUNTINGTON 
Perhaps the most significant example of the response of pul- 
monary architectural organization to aquatic adaptation is 
afforded by the lung of Hippopotamus liberiensis which, in con- 
trast to all the other typical artiodactyls with right eparterial 
bronchus derived from the trachea, possesses a bilaterally sym- 
metrical tree, each stembronchus furnishing an eparterial com- 
ponent. 
We have previously referred (p. 128) to lungs in which in the 
phylogenetic evolution this selective modification of bronchial 
structure has not been active, because the descendants of a more 
primitive lung-type, in their adaptation to an altered environ- 
ment, change only those parts of their organization which are 
directly affected by the new environmental conditions. If the 
lung is not one of the parts thus acted upon, the respiratory 
apparatus is not called upon to modify its structure. The pul- 
monary organization, both in the archeal evolutionary stage and 
in the later adaptations to new conditions, was, and remains, 
adequate for the required respiratory exchange. We found 
(p. 180) in this balance between lung-structure and specific en- 
vironmental factors the explanation of the apparent discrepancy 
of the bronchial tree of Tazxidea compared with that of the 
remaining Mustelidae, of the Hystricomorphs as against the rest 
of the rodent order, and of the atypical forms among the cetacea, 
if such really exist. 
The factors which, on the physiological interpretation of the 
comparative anatomical evidence, may be regarded as determining 
specifically pulmonary evolution, range themselves under the 
following headings: 
1. In its broadest lines the vertebrate lung emphasizes in its 
structure the effects of the enormous metabolic change which 
the increase of oxygenation has entailed in passing from the poi- 
kilothermal amphibians and reptiles to the homoeothermal birds 
and mammals. This finds its morphological expression in the 
neomorph development of the cranio-ventral pulmonary districts 
of the two higher classes, particularly in the acquisition of the 
eparterial system. On the other hand the higher lung-type of the 
more advanced reptilian forms depends largely upon the increased 
