100 GEO. S. HUNTINGTON 
These questions materially influenced in varying degrees the 
course of subsequent investigation. 
A critical analysis of the work in this field during the forty 
years following Aeby’s publication is, therefore, perhaps best 
undertaken by presenting, in place of a strictly chronological 
sequence, a grouping of the chief results obtained under provis- 
ional headings suggested by the main lines of thought concern- 
ing the phylogenetic history of the mammalian lung which deter- 
mined the trend of the investigations. It then appears that the 
five fundamental questions listed above elicited four main hypo- 
thetical considerations of the problem. The evidence, on 
which these theories were based, is common ground, but the 
conclusions reached are divergent. On the assumption of a 
common ancestral bronchial pattern of the mammalian lung, 
from which all extant types are descended by evolutionary modi- 
fications, the question arose which of the existing types conformed 
most closely to this promammalian bronchial tree, and hence 
represented the nearest approach to the hypothetical primitive 
mammalian lung. It appeared probable, on general genetic 
grounds, that the ancestral lung of the mammalia possessed 
more or less complete bilateral symmetry, and that hence the 
choice lay between the bilaterally symmetrical eparterial type 
(found in some pinnipede carnivores and cetaceans, the Cebidae 
among primates, certain rodents and perissodactyls, elephas, 
hyrax, etc.), and the bilateral symmetrical hyparterial bron- 
chial tree of some Hystricomorphs and of Taxidea. These con- 
siderations gave rise to three interpretations. 
I. REDUCTION THEORY 
Aeby (1878, 1880, 1882) 
D’Hardiviller (1896, 1897) 
Bremer (1904) 
Aeby assumed (2) that at one period in the evolutionary his- 
tory of the mammalian lung the organ possessed a bilaterally 
symmetrical bronchial ground-plan, comprising within its scope 
all the components found among the extant mammalian types. 
He held that this archeal ground-plan was preserved unchanged 
