PULMONARY EVOLUTION IN MAMMALIA W227 
monary element lost in the phylogenetic evolution of the mam- 
malian lung. This is confirmed by the occurrence of the adult 
bronchial variations of Lepus cited above (p. 104) in this con- 
nection. I feel that we are justified, on the basis of all of the 
actual evidence, in concluding that any mammalian lung which 
has once acquired a focal point of entodermal bronchial prolif- 
eration, retains the same in its structural organization, even 
against apparently unfavorable local and topographical condi- 
tions, as, e.g., in limitations of the available thoracic space. 
On the other hand, accepting Plesictis as the common ancestor 
of the modern mustelidae, it is possible to assume that the 
Plesictis-lung was organized on the bilateral symmetrical hypar- 
terial type, retained in only one of the extant descendants of 
to-day, Taxidea, while in all the other recent mustelidae, as far 
as determined, environmental changes have so affected the con- 
stitution of the germ-plasm as to produce the more advanced 
pulmonary type with the development of the right eparterial 
bronchus. All of our available evidence proves the mammalian 
lung to be extremely sensitive in its structural response to en- 
vironmental changes affecting respiration. Any environment 
capable of producing chromomeric alteration in the developing 
germeells of Plesictis must have affected equally and uniformly 
all the descendants derived from the same ancestor. 
The lung of Tazidea is physiologically as efficient in supplying 
the respiratory requirements of the organism it serves as are 
the lungs of the other mustelidae. Otherwise Taxidea would 
either have become extinct, or its lung would have adapted itself 
structurally to the conditions which induced the pulmonary dif- 
ferentiation in all the other mustelidae. The hypothesis here 
under discussion would therefore become untenable under its 
own terms, were it not permissible to analyze the evolutionary 
factor, which we described as environmental adaptation, more 
closely into its component parts. 
The specific, generic and ordinal differentiations of the extant 
mammalia appear clearly as the outcome of adaptations to a 
physical environment differing widely in the various forms of 
mammalian organization in respect to habitat, character of food, 
