190 GEO. S. HUNTINGTON 
Selection theory 
Summary 
The theory of selection is based on the morphological and 
physiological conditions which have combined in the evolution 
of the mammalian pulmonary organization. 
I. Morphogenetic principles. The phyletic interpretations of 
the extant types of bronchial architecture in the mammalia, and 
the homologization of the right and left bronchial components 
in the asymmetrical forms, does not depend upon the rearrange- 
ment of a fixed number of primary bronchi derived from a hypo- 
thetical archeal bronchial tree and shifted within this frame in the 
various types, much as chessmen are moved around on the 
board. It rests on the fact that the entire entodermal respira- 
tory anlage of the mammal has evolved on the basic pulmonary 
organization transmitted from its batrachio-reptilian ancestry, 
sharing in all the developmental potencies of the latter. It is 
not only unnecessary to suppose that extant reptilian types, if 
determined adequately, would yield an unbroken and closely 
graded series leading directly to the mammalian pulmonary or- 
ganization, but all the available phylogenetic, ontogenetic, and 
comparative evidence negatives such an assumption. 
It is probable, as outlined above, that the promammalia emerged 
from a reptilian stem with lungs corresponding approximately 
to the organs of the simpler modern lacertilia, or at most ad- 
vanced to the stage of pulmonary evolution occupied to-day by 
the more primitive paludal and littoral chelonia. Such a lung 
presented the pulmonary cavum still continuously lined by 
respiratory entoderm. The more complex and highly organized 
lungs of the marine chelonia and of the crocodilia developed the 
differentiation of the intrapulmonary bronchial system, second- 
arily engrafted on the primitive structure (cf. p. 160), and rep- 
resent specialized adaptations continuing the line of reptilian 
- development beyond the point at which the mammalian deriva- 
tion left the common ancestral stock. In this they follow the 
same underlying genetic laws which determine the further course 
of the mammalian evolution, but they lead to higher stages of 
