118 GEO. S. HUNTINGTON 
These are mostly characterized either by great bodily bulk, 
heavy musculature, rapid or long-continued locomotion, or by 
aquatic life, with its resulting effect on respiratory organization. 
The earliest mammalian or promammalian types, as stated, 
were exceedingly small, lacking bulk and muscular development of 
the modern ungulate. They were land forms, the marine adap- 
tations of the cetacea and sirenia were acquired in later geologi- 
cal periods. The pinnipede carnivora represent a still later and 
very limited adaptation to aquatic life. 
3. Among extant mammalia the ontogeny of the monotreme 
and marsupial lung (5, 22, 30, 32, 35) furnishes exceedingly 
important and definite evidence of the phylogenetic descent of 
the primitive mammalian respiratory tract from a reptilian 
ancestry. Comparison of early embryos in the egg or the uterine 
stages with the later pulmonary anlages after hatching or dur- 
ing the pouch period, clearly bridges the gap between adult 
reptilian and mammalian organization (cf. p. 192) but fails to 
show any eventual departure from the typical mammalian plan 
in the ontogeny of the bronchial tree in these forms. 
The monotremes in particular stand morphologically in closer 
relation to the promammalian reptilian ancestry than any other 
extant mammalian type. If the earliest mammalia already pos-: 
sessed the bilateral eparterial expansion of the bronchial tree, 
it is reasonable to expect its retention, if anywhere, in these 
most primitive members of the class. On the contrary the 
cranial segments of the monotreme lung are strikingly under- 
developed. Both Echidna and Platypus possess the dominant 
mammalian asymmetrical type of bronchial distribution. The 
very small eparterial bronchus is confined, except in variants, 
to the right side, supplying a small apical process of the right 
lung, which appears to be scarcely more than a rudimentary 
appendage to the lung-stem. The monotreme lung, instead of 
supporting the Reduction Theory, suggests rather the first un- 
folding of the cranial lung poles leading eventually to the 
extension of the neomorph eparterial districts. 
4. In completing this review of the Reduction Theory it is 
finally of interest to consider the grounds on which Aeby selected 
