124 GEO. S. HUNTINGTON 
They further suspend respiration for longer or shorter periods 
during submersion. In almost all forms special provision is 
made for the collection of the venous blood in large reservoirs 
during the respiratory intermissions, such as the huge hepatic 
caval sinus of the seals, and the enormous retiform plexus of the 
abdominal visceral and genito-urinary veins of Macrorhinus. 
When the animal emerges for breath the large amount of accumu- 
lated venous blood calling for oxygenation requires a rapid and 
complete exchange. In conformity with this demand the pul- 
monary architecture of these forms presents the most brilliant 
examples of the full bilateral and symmetrical development of 
the eparterial system of both lungs. In attempting to evaluate 
the significance of these facts they lead to the following con- 
clusions: 
A. Especially exacting functional demands are accompanied 
by modifications of the mammalian pulmonary organization 
through more extensive development of the eparterial bronchial 
districts. This extension of the ultimate respiratory area may 
occur only on the more favorably disposed right side (ef. p. 185), 
by utilizing a more cranial point of derivation of the right epar- 
terial bronchus, directly from the trachea cranial to the bifur- 
cation of the main tube (artiodactyls, sirenia, many cetaceans), 
or on both sides by the additional development of the left epar- 
terial trunk (perissodactyls, pinnipede carnivores, aquatic ro- 
dents, arboreal primates, Hippopotamus). 
B. Environmental factors have so changed the constitution of 
the germplasm as to transmit the resulting alteration in pul- 
monary type. It is probable that the influence of environment 
upon the developing germ cells, and through them on the prog- 
ress of evolution, has been underestimated in the past (Weis- 
mann). Variation of lung structure, especially the occurrence 
of mutations (28), may have been the sole, or at least the prin- 
cipal, phylogenetic factor which led forms possessing these modi- 
fications, and by virtue of them capable of greater pulmonary 
unfolding, to follow lines of environmental adaptation ending in 
the evolution of the marine, ungulate and aquatic types char- 
acterized in their extant descendants by the high development 
of the eparterial system. | 
