EVALUATION OF CAUSES IN RISE OF AMPHIBIANS * 421 



the development of the heart has proceeded farther and the two auricles 

 are completely separated, but there is still but a single ventricle. In 

 certain forms, as in Protopterus among dipnoans, there is a longitudinal 

 median ridge on the walls of the ventricle and partitions in the conus 

 arteriosus which serve further to guide the purer blood toward the head. 

 Another device, present in many reptiles, is for the ventricle to contract 

 in such a way as to assist in separating the two blood streams and driving 

 in advance the purer blood toward the head. 



Newly oxygenated blood should not, however, be sent to the liver before 

 having its oxygen used by the body; blood from the intestines must, on 

 the other hand, be sent to the liver. What, then, are the relations of the 

 circulation of the blood leaving the air-bladder, and do they fit with the 

 hypothesis that the original use of the air-bladder was as a supplemental 

 breathing organ? In answer, it is found that among the higher fishes 

 most of the veins from the air-bladder join the hepatic portal vein, but 

 more or fewer of them, especially those from the dorsal wall of the organ, 

 open into the posterior cardinals, and thus pass the blood directly into the 

 heart. They may, as in Polypterus, even join the veins leaving the liver. 

 In Neoceratodus the veins from the air-bladder unite to form a single 

 vessel which opens as a separate vein into the left auricle. Thus, in the 

 forms most primitive and making the largest use of the respiratory func- 

 tion, more of the blood is sent from the air-bladder directly to the heart 

 and less to the liver. 



Finally, in figure 1, D, is shown the completely efficient system of a 

 four-chambered heart with its complete separation of the two blood 

 streams, independently evolved in the two warm-blooded classes of verte- 

 brates — birds and mammals. Crocodiles among reptiles have achieved 

 the same plan, though the dorsal aorta receives blood from both ventri- 

 cles and therefore still supplies mixed blood to the posterior parts of the 

 body. This four-chambered heart is equivalent to two two-chambered 

 hearts. Consequently in the development of air-breathing by means of' 

 lungs a second heart has had to be ^evolved and added to the original 

 selachian system, but the development of the second has lagged long 

 behind the rise of lungs. The essential similarity of the system inde- 

 pendently attained in birds and mammals points to its necessity in that 

 competition in life which demands sustained activity. By comparison 

 the hearts of the lower air-breathing vertebrates are seen to show mere 

 makeshift devices. 



If it be sought to place the circulatory system of a lung-fish, such as 

 Neoceratodus, in its evolutionary position between that of the selachian 

 and that of the mammal, it is seen that in the retention of the gills it is 



