a2, GEO. S. HUNTINGTON 
This view is supported by the following considerations: 
1. In the first place it does away with the untenable assump- 
tion of the Reduction Theory that the promammalia had ac- 
quired a higher and more efficient pulmonary organization than 
the one exhibited at the present day by the vast majority of their 
descendants, in whom pulmonary reduction on the left side, and 
in a few forms on both sides, is supposed to have established the 
hyparterial condition secondarily. 
2. It is very clear that the mammalian lung is structurally on 
the ascent, rather than on the decline. It can in no sense be 
grouped with the decadent structures of vertebrate organization. 
Morphologically it does not: represent the direct continuation of 
an old respiratory organization, but a new structure replacing the 
archeal branchial apparatus, which in its turn was the termination 
of the long evolutionary path traversed by the vertebrate respira- 
tory tract in its many antecedent phyletic stages (cf. 29, p. 179). 
It is contrary to all genetic principles underlying evolution to 
consider a neomorph of this character capable of entering the 
organic complex in a state of such advanced development as to 
call for subsequent reduction of the number and extent of its 
components. The assumption of mutation is clearly not applic- 
able under the premises, for a mutant requires a pre-existing 
structure in which the change becomes manifest. 
3. The functional interpretation of the comparative ana- 
tomical evidence bearing on the question speaks for the progres- 
sive unfolding of the mammalian respiratory area, especially in 
its cranial sectors, in direct ratio to increased functional demand 
resulting from specific environmental adaptations leading to the 
establishment of the eparterial district in the more highly 
organized lungs. 
The ungulates in general are represented by animals of mas- 
sive bulk and heavily muscled. Many are endowed with the 
capacity for rapid and long continued motion, calling for in- 
creased tissue combustion and a rapid respiratory exchange. 
The members of the group have acquired a marked expansion 
in their pulmonary development. 
