PULMONARY EVOLUTION IN MAMMALIA LP 
of the mammalian eparterial system by the tenets of the Reduc- 
tion Theory of pulmonary evolution in the mammalia. I con- 
sider that the same should be definitely abandoned in the present 
stage of the problem. 
II]. EXTENSION AND MIGRATION THEORIES 
Willach (1888), Young and Robinson (’89). 
Ewart (1889), Zumstein (’89), Robinson (’89). 
Narath (1892), Huntington (98). 
Narath (1901), Blisnianskaja (04). 
The inconsistencies in Aeby’s Reduction Theory discussed in 
the preceding section soon led observers to follow two lines of 
thought which were necessarily at that period linked more or less 
closely together. One of these, which can be defined as the 
Extension Theory, sought to approach the problem on its more 
purely phyletic side, while the second, as the Migration Theory, 
dealt with the morphogenetic factors supposed to be operative 
in the development of the different architectural types of the 
bronchial tree encountered in the mammalia. 
A. EXTENSION THEORY 
On the basis of the phylo- and ontogenetic facts above consid- 
ered, and supported by additional observations, I advocated, in 
a paper published in 1898, (25) a reversal of Aeby’s Reduction 
Theory. and regarded the bilateral hyparterial bronchial tree 
as the starting point instead of the decadent termination of a 
phyletic series in which the bilateral eparterial type represented 
the full development of an evolutionary unfolding of the mam- 
malian lung in response to varying degrees of functional demand 
placed upon the respiratory tract. 
If either of the two bilaterally symmetrical bronchial trees is 
still to be considered the primitive mammalian form with which 
the others stand in direct genetic relation, there are weighty 
reasons for regarding the bilateral hyparterial type as the most 
archeal bronchial organization among extant mammalia and as 
approaching nearer both to modern reptilian structure and to 
- the hypothetical ancestral pattern of the promammalian lung. 
