188 GEO. S. HUNTINGTON 
ing the frequently observed cardinal variation of a left subclavian 
artery arising from the normal thoracic aorta, with the rare case 
in which the same variation of the primary arterial branches is 
combined with a right-sided thoracic aorta and the retention of 
a left Botallian Duct. 
The facts observed during the ontogeny of the dominant  pul- 
monary type in the mammalia account for the conditions found 
in the adult. Any extensive series of typical mammalian pul- 
monary corrosions will show that in the great majority the car- 
diac rotation has carried the origin of the main pulmonary trunk 
so far ventrad and to the left that the primary division of the 
vessel takes place to the left of the trachea or of its bifurcation. 
The longer right pulmonary artery hence crosses the ventral sur- 
face of the lower end of the trachea or its bifurcation from left to 
right. In descending to the point of its intersection with the 
dorso-lateral surface of the stembronchus in the angle formed by 
the first ventral bronchus with the latter, the artery lies chiefly 
ventral to the proximal portion of the right stembronchus, be- 
tween the origin of its first ventral branch and the tracheal 
bifureation. The cranio-lateral surface of the stembronchus thus 
left free is occupied by the origin of the right eparterial bronchus. 
The shorter left pulmonary trunk turns almost sagittally dorso- 
caudad, covering directly the cranio-lateral circumference of the 
left stembronchus in the interval between the tracheal bifurca- 
tion and the origin of the first ventral hyparterial bronchus. 
The artery thus occupies the larger part of the segment which 
would be otherwise available for left eparterial development. 
If the general premises of the selection theory are well taken 
and if hence the eparterial components of the bronchial tree 
are secondarily engrafted during evolution on the archeal lung- 
stem, the mutual relations of the latter were already established 
at a time when additional cranio-ventral extension of the lung 
was inaugurated. The eparterial acquisition must fit into and 
become adapted to already existing conditions, which are more 
favorable on the right as compared with the left side. 
I consider these facts as chiefly responsible for the preponder- 
ant development of the eparterial bronchus only on the right side, 
