HIGHWAY OF ANIMAL EVOLUTION 229 



the heart, which supplies the driving power for the cir- 

 culation of the blood, and the main arterial channels 

 which supply the body. On the other hand, the re- 

 spiratory currents of water, entering the mouth and 

 passing out through the necessarily large gill clefts, 

 seriously interfere with the passage of foods through 

 the mouth to the stomach, and with the usage of this 

 part of the alimentary canal for digestive purposes. 



But when new and larger blood channels were 

 opened up, leading into the air bladder, a new respira- 

 tory organ, the lungs, was established in a safer and 

 better functional position. Lungs were then substi- 

 tuted for gills; a double circulation for a single one; 

 and the heart and the general arterial system thereby 

 acquired greatly increased powers of service. This 

 relatively simple readjustment in the direction of the 

 blood current which, with other accompanying 

 changes, made it possible separately to regulate the 

 blood pressure in the lungs, and in that of the general 

 arterial system, was of great functional importance. It 

 is the chief cooperative event which marks the transi- 

 tion from fishes to the amphibia and reptiles; and the 

 register of this change is still preserved in the embryos 

 of all their descendants. But its effects did not stop 

 there; it also opened the way for a terrestrial, air- 

 breathing life, and for the hot, even-tempered blood 

 of the higher vertebrates. The subsequent changes in 

 the direction and volume of the blood currents, and the 

 influence of the new mode of impact on the walls, 

 valves, and openings of the heart, were doubtless im- 

 portant physical agencies in completing the four cham- 

 bered heart which came later, making it possible to 

 deliver the rich supply of arterial blood directly to 



