PULMONARY EVOLUTION IN MAMMALIA 193 
structure which can manifest itself under suitable conditions by 
the proliferation of respiratory buds from any part of its extent. 
This plastic period, constituting a distinct phase of aplacental 
development, and rehearsed briefly in the early ontogenetic 
stages of the higher groups, forms the phylogenetic basis for the 
gradual evolution of the divergent types of the mammalian 
bronchial tree. 
III. The occurrence of individual bronchial variants within 
a species, both secondary and cardinal in character, further con- 
clusively demonstrates the existence of a period in the individual 
development during which the inherited ancestral bronchial 
pattern is capable of alteration, producing variations of greater 
or lesser extent. . 
Man is, in his pulmonary organization as in the other details 
of his structure, the best known mammal by reason of the rela- 
tively enormous number of individuals examined anatomically 
and the accumulation of the records pertaining to them. The 
human bronchial tree is of the dominant mammalian asymmet- 
rical type with the eparterial distribution developed only on the 
right side. Yet a number of instances of both the bilateral 
hyparterial and the bilateral eparterial type are recorded in 
man. 
Similar or greater variability is found among the lower mam- 
malia. It is usually less striking only on account of the smaller 
number of individuals of each species examined. Some forms 
are, however, exceptionally prone to bronchial variability. 
The Hystricomorph rodents constitute an important group in 
which it is clear that pulmonary evolution is actively progressing 
at the present time, with the bronchial organization in a state 
of unstable morphological equilibrium and presenting frequent 
mutant variations (28). 
A series of ten individuals of Hrethizon dorsatus contains rep- 
resentatives of both the bilateral eparterial and hyparterial 
symmetrical bronchial types, of the dominant mammalian asym- 
metrical right eparterial form, and of a transitional variant 
which appears as the normal fixed type in Sphingurus prehensilis, 
another member of the Hystricomorph group. 
THE AMERICAN JOURNAL OF ANATOMY, VOL. 27, NO. 2 
