PULMONARY EVOLUTION IN MAMMALIA 183 
demands for increased pulmonary area the new arrival appro- 
priates a still more proximal portion of the stembronchus. The 
tracheal bifurcation, the trachea itself to a varying height, fur- 
nish the epithelial fundament for increased respiratory extension 
in new points of eparterial bronchial derivation from the original 
stem-line. 
In all cases the available intrathoracic range of eparterial 
extension is more or less restricted, so that the opportunity or 
necessity for the development of more than a single eparterial 
bronchus does not arise. The anlage for it has only to bud far 
enough away from its nearest hyparterial neighbor to obtain all 
the space requisite or available for its development. Still the 
possibility for the unfolding of more than one eparterial bud is 
inherent in the organization, as shown by instances of double 
eparterial bronchi in a few forms (Giraffa, Camelus, Auchenia, 
Delphinus, Myrmecophaga), in which a large tracheal eparterial 
bronchus is accompanied by a smaller eparterial contribution. 
This is usually derived from the right stem bronchus, but in 
Giraffa arises from the trachea caudal to the main eparterial 
trunk. 
Factors responsible for the prevalent asymmetry of the mammalian 
lung 
In the dominant type of mammalian intrapulmonary archi- 
tecture eparterial development is confined to the right lung, with 
the eparterial bronchus derived from the right stembronchus. 
The resulting pulmonary asymmetry is encountered in at least 
95 per cent of the mammalian genera and species whose bron- 
chial trees are recorded, and hence calls for detailed considera- 
tion in any critical analysis of mammalian pulmonary evolution. 
The question has long attracted attention and the etiological 
factors held responsible for the prevalent condition have been 
discussed in a general way. 
The encroachment of the heart on the left half of the thoracic 
space is evidently the cause of the relative reduction and loss of 
volume of the left as compared with the right lung. This is 
