PULMONARY EVOLUTION IN MAMMALIA 7 
In the advance of the mammalian evolution beyond the early 
tertiary types, the general size of the body appears to increase, 
sometimes markedly, in the more highly specialized forms. In 
the middle eocene the marine adaptations appear, developing 
into the cetacea and sirenia. The pinnipede carnivora repre- 
sent a later branching, which never played an important part in 
mammalian evolution. The view that all aquatic vertebrates, 
except the fishes, have been derived from terrestrial ancestors, 
rests upon cumulative. evidence. Fully aquatic mamma’s, 
apart from their high specialization, are always degenerate in 
some respects. Gregory, in a personal communication, sums up 
the known facts of placental ancestry as follows: 
In brief there is much evidence for the view that the remote ances- 
tors of all the modern placental orders were small animals, perhaps of 
the size of a large opossum, with very low brains, small brain-case, 
Spiess, 
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muzzle and olefactory parts large, limbs short, stout, heavily muscled, 
hands and feet short, pentdactyl and of more or less grasping type; 
diet perhaps insects, small birds and reptiles, and vegetation; habitat 
more or less arboreal. 
stout jaws, high sagittal crest, heavy molars, dentition 
Palaeontological evidence, therefore, as far as it indirectly 
_ becomes tangent to the mammalian pulmonary problem, weighs 
strongly against the Reduction Theory. If we group the living 
mamalian types with highly developed or bilateral eparterial 
bronchial districts together, we obtain the following general 
results: 
Terrestrial forms: 
Perissodactyla, with the exception of Tapirus. 
Artiodactyla. 
Elephas. 
Hyrax. 
Some Platyrrhine Primates (Cebidae). 
Aquatic forms: 
Most Cetacea. 
Sirenia. 
Pinnipede Carnivora. 
Certain aquatic Rodentia. 
Hippopotamus. 
