PULMONARY EVOLUTION IN MAMMALIA 133 
B. MIGRATION THEORY 
The concept of the morphological equivalency of the right and 
left lungs, masked in the numerically preponderant asymmetrical 
types by differences in the topographical relations of the bron- 
chial components, led early to attempts at bronchial homo- 
logization, and formed the basis of the Theory of bronchial 
Migration. 
This Theory assumes an archeal common mammalian ground- 
plan of bilaterally symmetrical bronchial organization with a 
fixed number of main derivatives from the stembronchus, whose 
side branches, under the stimulus of progressive evolution, are 
endowed with the capacity of moving or migrating as a whole 
from their primitive site to new points of origin from the stem- 
bronchus, thus acquiring a greater degree of independence and 
altering the type of the entire bronchial tree. If the shifts occur 
in the same direction and to the same extent on both sides, the 
resulting bronchial tree remains in the bilaterally symmetrical 
group. If migration is confined to one lung, or carried further 
on one than on the opposite side, the resulting bronchial tree 
becomes asymmetrical as in the dominating mammalian type 
with eparterial bronchial development limited to the right lung. 
By carrying this process of ‘Migration’ to a varying level on the 
stembronchus of one or both sides, or extending it to the right side 
of the trachea, the different types of bronchial distribution 
encountered in extant mammalia are produced. 
Aeby in 1880 already entertained the general concept of a 
phylogenetic shift or migration of bronchial components as the 
evolutionary factor responsible for the production of the five 
types of the mammalian bronchial tree determined by him. He 
does not elaborate this idea in any detail, but it was evidently 
clearly in his mind when he ascribed to his eparterial bronchus a 
very pronounced degree of ‘Wanderungsfihigkeit.’ The thought 
is further expressed in several passages of his work (2, pp. 6,7). 
Willach (37), in 1888, formulated the hypothesis more com- 
pletely, and to him must be assigned historical priority as the 
founder of the formal Migration Theory. He included in his 
