PULMONARY EVOLUTION IN MAMMALIA 131 
of this whole. Confronted by a variety of mutually incompat- 
ible opportunities, each pointing a divergent evolutionary path, 
the choice of one of these, with the necessary exclusion of the 
remainder, will determine the resulting structural type. Thus 
the components of the organism which come into direct relation 
with a diversified environment will adapt themselves only to 
certain factors of the same, with the resulting production of 
varied and divergent forms, while the lung, which encounters 
the uniform and practically unvarying influence of its specific 
factor, the air, will correspondingly appear as a uniform structure 
in all the different types. 
The following conclusions can be based on these considerations 
in the special problem here discussed: 
While the mustelidae as a group, including Taxidea, adapt 
themselves to their special environment in regard to all parts 
of the organism in direct relation to the same, the resulting 
- adaptations and differentiations did not alter the status of the 
lung, because the environmental factors did not include the special 
conditions determining specific changes in the pulmonary archi- 
tecture. They thus become welded into the homogeneous taxo- 
nomic group of the modern mustelidae in all the external char- 
acters of body-structure, alimentation, habitat, locomotion, etc., 
but the intrinsic pulmonary organization was left untouched by 
any environmental influence directed specifically toward struc- 
tural changes in the architecture of the lung, beyond that com- 
mon tothe mammalian type in general. It is conceivable that 
the direct forebear of Taxidea had retained a more primitive 
type of bronchial organization than the remaining contemporary 
forms which had already advanced to the acquisition of the 
right eparterial bronchus. 
In the course of further evolution no additional factor was in- 
troduced to alter the initial discrepancy between the two pul- 
monary types, which has thus become perpetuated in their 
present representatives. 
This appears to be the most available interpretation account- 
ing for the existence of the closely knit zoological group of the 
modern Mustelidae, consisting of forms which agree in the de- 
