132 GEO. S. HUNTINGTON 
tails of their general organization, but include one member, 
Taxidea, differing from the remainder in the structure of its 
bronchial tree. 
These considerations led me to regard the bilateral hyparterial 
bronchial tree as the archeal type, forming the basis of the 
Extension Theory, upon which the remaining mammalian bron- 
chial organizations are built, rather than to begin at the top, 
and by successive detachment of branches, arrive at the bottom, 
as postulated by the Reduction Theory. 
I am more than ever convinced by the additional phylogenetic 
evidence which the intervening years have brought to the study 
of the problem that the basic concept outlined above contains 
the seed of the correct interpretation of the evolutionary process 
responsible for the extant types of the mammalian lung structure, 
but I have abandoned my original views regarding the genetic 
stages involved. In the earlier period of this study the idea of 
the migration of pulmonary components was beginning to make 
headway and appealed to me as affording the most available 
explanation of the steps in pulmonary evolution upon which to 
base the Extension Theory. Subsequent to the publication of 
the same, continued further comparative anatomical and onto- 
genetic study of the problem led to a radical revision of my 
conception as to the morphogenesis involved, and I abandoned 
the Migratory Theory for the view which impressed me as the 
only logical deduction warranted by all the facts, and which, 
for want of a better term, can be briefly designated as the evo- 
lution of bronchial types by selection or adaptation. This 
Selective Theory is fully expounded in the final section of this 
paper. Before proceeding to its consideration it is proper, at 
this level of the historical review, to present the second thought 
linked to the theory of pulmonary evolution, viz.: 
