136 GEO. S. HUNTINGTON 
right lung. A similar accessory bronchus may develop on the 
left side as the left cardiac bronchus to a left azygos lobe, or to 
the corresponding portion of the left lung stem. 
The thought underlying this concept of mammalian bronchial 
evolution is best expressed in Narath’s own words: “‘Es herrscht 
ja tiberall im Bronchialbaum das Gesetz der Abspaltung.”’ 
This condensed account embodies the chief dogmata of the 
Migration Theory. It is psychologically of interest to consider 
briefly the reasons for the ready acceptance of this idea and its 
almost universal adoption in scientific and educational circles 
to-day. 
In the first place the enormous numerical preponderance of 
the dominant type II* among living mammalia imparts to the four 
remaining architectonic types the character of aberrations. By 
reason of their bilateral symmetry they give the impression of 
being either earlier, more primitive forms, or they suggest a rela- 
tion to type I*, as representing deductions from the same or 
additions to type III. No observer who has studied large 
series of pulmonary corrosion preparations can fail to recognize 
the close architectural gradations of the bronchial tree in differ- 
ent forms. Without considering the variations within a single 
species, which are often numerous and highly suggestive, when 
many individuals are compared, it is easy to form long series 
whose links pass into each other by almost insensible degrees, 
and thus join the extreme examples of a type at either end of 
the line. It is natural to regard these transitions as pointing 
the way to the phylogenetic derivation of a given type from a 
preceding stage by a shift or migration of some of its bron- 
chial components. Narath furnishes an excellent illustration of 
this mental process. In support of his contention that the dorsal 
derivatives of the stembronchus were originally side branches of 
the ventral bronchi which have migrated onto the stembronchus 
itself, he says (33, p. 311): 
Man kann sich bei manchen schén gelungenen Injectionspraparaten 
kaum des Gedankens erwehren dass der dorsale Bronchus ein Art- 
genosse der externen Seiteniiste der Ventralbronchien ist und dass er 
gewissermassen als Vorposten vorgeschoben wurde auf den Stamm- 
bronchus. 
