EVALUATION OF CAUSES IN RISE OF AMPHIBIANS 419 



We have seen occasion in the present study, however, to reverse certain 

 of these supposed causes. The fishes appear to have migrated not from 

 the oceans to the rivers, but in the contrary direction, yet the ancient 

 cyclostomes show no trace of an air-bladder, though many of them return 

 to the fresh waters for breeding. It is doubtful if the selachians ever 

 possessed this organ when in their fresh-water habitat, since they now 

 preserve no remnant of it, even in a larval state. A small caecum em- 

 bedded in the dorsal wall of the oesophagus of certain sharks may pos- 

 sibly, however, be homologous. It is further seen that the mere lure of 

 food outside of the normal habitat is hardly competent to transform the 

 physiology of a dominant race which could already find adequate suste- 

 nance. For such a great organic change there would appear to be neces- 

 sary a positive pressure tending to drive the race out of its old habitat, 

 not merely an attraction — a mere negative pressure of environment. 

 That driving pressure was apparently the compulsion of the inorganic 

 environment, not the pressure from organic foes. This will appear more 

 clearly under the consideration of the next topic. 



ARTERIAL AND RESPIRATORY SYSTEMS OF FISHES AND AMPHIBIANS 



In figure 1, A, is given a diagrammatic representation of the plan of 

 the selachian circulation, the type from which has arisen that of all 

 higher vertebrates. It is seen that all the blood is forced from the heart 

 through the gills, where it is oxygenated and passes thence to the body. 

 That supplying the brain is taken independently from the gills and 

 passes over a proportionately greater gill surface than that destined for 

 the remainder of the body. This system of circulation is simple, involv- 

 ing only a two-chambered heart, but is highly efficient, only oxygenated 

 blood being sent to the body, only waste-laden blood returned to the 

 heart. Its efficiency is measured by the power and sustained activity 

 exhibited by many fishes and the rapidity of asphyxiation when they are 

 removed from water. 



In figure 1, B, is shown the circulatory system of ISTeoceratodus, the 

 most primitive in its circulation of the few remaining dipnoans. It also 

 represents a certain embryonic stage in the development of the amphibian 

 circulation. It is seen that the air-bladder, functioning as a lung, re- 

 ceives blood from the last one only of the efferent gill arteries. If the 

 auricle, ventricle, and conus arteriosus were of the same simplicity as in 

 the selachian heart, this would mean that only a fraction of the blood 

 would be sent to the lung after each passage through the heart. If the 

 gills were not functional, the blood sent to the head and body would in 

 that case be mostly unoxygenated. The system would be so inefficient 



