PULMONARY EVOLUTION IN MAMMALIA 155 
In all these wanderings the bronchus retained its own individual 
character, always representing the same Odysseus-like phyletic 
element of the ancestral bronchial tree. No matter into what 
foreign territory the voyage leads, the germinal cellgroup aways 
carries with it the invisible mystic impulse to develop into a 
specific bronchus. On the other hand it is evident that the actual 
facts are adequately and completely met by simply recognizing 
that space disposition within the thorax permitted and increased 
respiratory requirements demanded, in the evolution of the 
Proungulate type, the selection of a more cranially located point 
on the primitive entodermal pulmonary tube for the origin of 
the respiratory bud corresponding to the bronchial eparterial 
area of the dominant mammalian type, even if this origin is thus 
made to fall within the tracheal segment. Embryonal muta- 
tions probably played a deciding réle in the evolutionary process 
involved. 
I believe there can be no question as to which of these two 
hypotheses is the more tenable and conforms more clearly to the 
facts. 
Flint (21, p. 35) who has furnished us with a most careful and 
detailed developmental history of the lung in a form (pig) with 
right eparterial tracheal bronchus (his L. 1) says: 
If we turn for a moment to the consideration of the origin of L. 1, 
we find the bronchus is a trifle more precocious, but practically simul- 
taneous with the second lateral branch (our V') in its origin. It is 
separated from Lateral 2 by a considerable distance. If the views of 
Willach and Narath were correct, this branch should not appear until 
later, and should be traceable, step by step, from the bud forming 
right Lateral 2 to its final position on the trachea. Its direction is 
practically lateralwards with a scarcely visible tendency to point ven- 
tralwards. It would not then, from the topography of its origin, bear 
any analogy to a dorsal bronchus. From this distinctly lateral posi- 
tion of its origin, I have classed it among the lateral group of bronchi, 
although, in its subsequent growth, one of its branches extends down 
into the dorsal region giving the bronchus a certain superficial resemb- 
lance to that series. On the other hand, the lower lateral elements 
grow ventralwards in the later embryonic stages and thus also lose 
their early strictly lateral character. This much is certain; if L. 1 
arises phylogenetically from the dorsal group, a view for which there 
is no convincing proof, absolutely all trace of the migration is lost in 
