PULMONARY EVOLUTION IN MAMMALIA 191 
pulmonary development distinctly reptilian in the character of 
its details and divergent from those attained by the mammalia. 
The archeal promammalian lung-tube possessed, like its lacer- 
_tilian prototype, the potency of differentiating conductory lines 
and peripheral respiratory areas by a selective development of 
bronchial buds from any point on the entodermal surface, fol- 
lowing the morphogenetic lines detailed above for the reptile. 
The number of these primary foci of heightened mitotic activity 
leading to epithelial proliferation, and their position relative to 
each other and to the axial pulmonary canal, determined the 
bronchial type of distribution. They must have from their in- 
ception represented the sum of the reactions of the milieu on a 
pulmonary organization which, at its emergence from the rep- 
tilian stem, was in a highly plastic condition and responsive to 
new environmental and metabolic factors. It is hence quite 
possible that the various types of the modern mammalian lung 
owe their diversity in part to polyphyletic derivation. The 
bronchial organization best adapted to respiratory conditions 
obtaining at any one evolutionary phase for each: mammalian 
form became fixed for that form by heredity as long as the 
respiratory factor of the environment remained unchanged. 
The extant ordinal, generic and specific types are the result of 
the transmission of selective patterns to the descendants. 
All the available evidence goes to show that in certain mam- 
malian groups the more primitive bronchial types have, during 
the progress of this evolution, undergone modifications, some- 
times of wide import, in response to altered environmental and 
functional demands. The opportunity for the continued devel- 
opment of such secondarily acquired adaptations rests in the per- 
sistent morphogenetic plasticity of the bronchial system and the 
retention by it of the potency, derived from the primitive ances- 
tral entodermal lung-tube, of inaugurating additional or atypical 
points of epithelial activity under the appropriate stimuli, leading 
to the production of neomorph bronchial components. 
I base these premises on three sets of facts: 
I. The above outlined phyletic development of the bronchial 
system in the vertebrate series. 
