PULMONARY EVOLUTION IN MAMMALIA 171 
Phylogenetic migration does not imply the bodily transplan- 
tation of a bronchial anlage. It means the establishment of a 
new focal point of bronchial epithelial proliferation whose prod- 
uct is a neomorph replacing the archeal bronchus, and, from the 
vantage-point of its new relation to the bronchial tree, enlarging 
the former’s area of respiratory distribution and thus enhancing 
its physiological value to the organism. The older bronchus 
thus replaced loses its earlier significance, becomes reduced, and 
either disappears or is continued as a subordinated side-branch. 
A bronchus per se does not travel. The phyletic impulse to 
inaugurate a new and more advantageously located bronchial 
derivative in its place from whatever point of the universally 
available entoderm is best adapted to the purpose, does travel. 
This sums up the essential difference between the ‘migratory’ 
and ‘selective’ theory of bronchial evolution. 
The theoretical example above adduced could of course not be 
visualized in the long line of a single phyletic series with types 
transmitted by direct inheritance. The evolutionary periods 
required for the accomplishment of such changes exceed by far 
the utmost limit of preservation of non-fossilized organic tissue. 
An archeal type has perished ages before we recognize the result 
of its evolution in the modern inheritors. But while the un- 
broken phyletic chain of mammalian pulmonary evolution in the 
line of direct descent is not actually available, its reconstruction 
is made possible to a large extent by the comparative analysis 
of extant types and by ontogenetic data. Thus, for example, the 
pulmonary problem receives much elucidation from the detailed 
study of the lung in groups like the following: 
Perissodactyls in comparison with Tapirus. 
Hyrax and the Proboscideae. 
Ungulates, Sirenia, some Cetaceans. 
Relation of Catarrhine and Platyrrhine Primates. 
Terrestrial and Aquatic Rodents. 
Hystricomorphs. 
Mustelidae and Taxidea. 
Giraffa and the Camelidae. 
Hippopotamus in comparison with the remaining Artiodactyls. 
