PULMONARY EVOLUTION IN MAMMALIA 199 
In the latter type all the accessory modifications are in the 
direction of storing CO, until the same can be exchanged in large 
quantities and rapidly during the resumption of respiration 
(Caval sinuses of Phoca and Manatus, abdominal venous plex- 
uses of Macrorhinus). The lung therefore must be so organized 
as to meet this intermittent demand for high efficiency, although 
not called upon in the intervals of respiratory suspension. The 
lung acquires the morphological complexity corresponding to 
the highest degree of efficiency which can be demanded in any 
given form under the normal environmental conditions. 
CONCLUSION 
The search for the elusive hypothetical promammalian 
ground-plan of bronchial architecture has terminated somewhat 
like the hunt for the Philosopher’s Stone. 
The mechanistic concept of a definite and crystallized archeal 
bronchial tree, from which all extant mammalian types are de- 
rived by modifications of the pattern through migration, does 
not exist in the commonly predicated form of a concrete and 
fixed morphological entity. It had no reality in any definite 
ancestral structure, save in the sense that the primitive reptilian 
lung and its phyletic derivative, the mammalian entodermal pul- 
monary anlage, both retain the potency of development by selec- 
tion into the type demanded by the environment. In place of 
the mythical common ancestral bronchial tree appears a living 
plastic organization, responsive to the changing demands of 
biological evolution and replete with answers to the modern 
problems of morphogenetic inquiry. 
