156 GEO. S. HUNTINGTON 
the pig. It originates like one of the lateral bronchi and subsequently 
develops its superficial resemblance to the dorsal elements. What- 
ever support for the relationship of the bronchus to the dorsal series, 
can be drawn from this fact, is multiplied by the behavior of a lateral 
branch of left Z. 2, which does exactly the same thing in an adaptative 
process on the part of the bronchus to a relatively unobstructed 
environment. 
With the artiodactyls and the kindred bronchial types the 
Migration Theory definitely collapses and the field is cleared 
phylogenetically and ontogenetically of the speculations which 
might be founded on a misinterpretation of less clear-cut con- 
ditions presented by other mammalian types. 
III. SELECTION THEORY 
The concluding section of this paper presents the viewpoint 
which has impressed itself upon me during these studies. If I 
dignify these deductions by the formal designation of a theory 
this is done solely with the purpose of establishing more sharply 
the contrast between them and the views considered in the pre- 
ceding pages. Actually they do not constitute a theory so much 
as an attempt to collect, coordinate and interpret as far as pos- 
sible all the facts accessible to me bearing upon the problem of 
the phyletic history of the Mammalian Lung. At the outset 
we are confronted by two questions, upon the answer to which 
depends the entire interpretation of the mammalian bronchial 
system and of the correlated organization of the lung. 
Does a careful consideration of all the established facts now in 
our possession warrant us in continuing to maintain the hypoth- 
esis of a single monophyletic primitive ground plan of mammalian 
bronchial architecture and to derive all extant types from the 
same by successive modifications? Or are we on the contrary 
dealing with the results of adaptation of the primitive vertebrate 
lung to conditions which have varied widely in different mam- 
malian prototypes according to thoracic structure and space 
disposition in the cranial portion of the body cavity, activity of 
respiratory exchange, mode of locomotion, general environment, 
and many other determining factors which have resulted in the 
