PULMONARY EVOLUTION IN MAMMALIA 169 
make up a topographical pulmonary district ordinarily served 
by a single bronchus. This might be indicated in the present 
example by the simultaneous development of both A and C 
(fig. 11%). 
Phylogenetic Migration 
To hold to the example of the right cardiac lobe suggested by 
the above schematic illustration, we will suppose that we are 
dealing with three individuals of the same mammalian species, 
and that the further examination of a large number of additional 
individuals of this species showed that the types 1, 2 and 3 (fig. 
10) occurred in approximately equal proportions, i.e., that a 
third of the total number examined carried, type 1, a third type 
2, and a third type 3. We could then conclude that no one of the 
three types held a distinct advantage over either of the others, 
and that all three have been transmitted by inheritance in equal 
proportions. If, on the other hand, we examined the same num- 
ber of individuals of a species allied to the preceding, and found 
type 1 in 90 per cent, type 2 in 18 per cent, and type 3 in only 
2 per cent, we would be entitled to the conclusion that the re- 
lations of the peripheral area X to the lung as a whole had in the 
second species undergone some change, as compared with the 
first, through environmental, topographical or other develop- 
mental factors, which rendered a bronchus of the type A of dis- 
tinct advantage to the individual possessing it, without barring 
altogether the occasional development of the bronchi B and C. 
For the second species type 1 has become dominant. 
Let us assume further that we are dealing with a larger mam- 
malian group of ordinal or subordinal rank, and that the indi- 
vidual genera composing the same not only possess so many 
distinctive morphological characters in common as to warrant 
the assumption of their genetic kinship, but that these common 
characters group themselves into an evident line of phyletic 
successions, placing the older and more primitive types at the 
beginning, the more recent forms at the end of this evolutionary 
line. 
If such material were available, and if we had the opportunity 
of carefully analyzing the details of the pulmonary organization 
