EVALUATION OF CAUSES IN RISE OF AMPHIBIANS 427 



trils ; the gill openings would become useless and retrogress to a temporary 

 embryonic feature. 



The absence of gill openings in air-breathing vertebrates, after the 

 early embryonic stages, although it thus has interesting suggestions, can 

 not be used, however, as a conclusive argument concerning the original 

 location of the accessory organs of respiration, since this being once 

 adopted, the other possible lines of evolution have not had opportunity to 

 be demonstrated by nature. This particular point is only of interest, 

 therefore, in consideration of what might have been, but never did come 

 to be. 



Let us turn back to the conclusions which may be drawn from the pre- 

 ceding lines of evidence bearing on the actual evolution of lungs. The 

 several arguments point backward, like convergent finger-boards, to an 

 early stage which has no living representatives ; but we may draw a sketch 

 from these converging probabilities. They indicate in the earliest ganoid 

 fishes, possibly in Silurian time, an initial habit of swallowing air as a 

 supplemental aid to respiration, necessary whenever the land waters be- 

 came subnormal in dissolved oxygen. This habit grew with exaggeration 

 of these environmental conditions and with increasing organic efficiency 

 in this direct absorption of oxygen. The upper part of the intestinal 

 canal would, in swallowing, have first contact with the air; would take 

 out most of the oxygen and become specialized for this purpose. By the 

 localization of respiration over this part of the canal, the necessity would 

 be avoided of the oxygenated blood passing into the intestinal veins with 

 the absorbed food products and thence to the liver. Greater need of 

 respiration would lead to enlargement of the surface, resulting in the 

 development of a sacculated diverticulum from the oesophagus. Mus- 

 cular control would follow, permitting the rhythmic intake and expul- 

 sion of air. It would now no longer need to pass through the intestine, 

 but would more readily be regurgitated through the mouth. 



When the supplemental use of air became more important than water- 

 breathing and at last became imperative, fundamental changes would 

 have to take place in order that somewhat more impure blood should be 

 sent to the lungs and somewhat less impure blood should be sent to the 

 head and body. These changes would have to be simultaneous through- 

 out the circulatory system and require that the arterial connection of 

 the lungs must be shifted forward from the cceliac artery, and that a 

 separate vein must pass directly from the lungs to the heart; the heart 

 also must be modified so as to keep the two blood streams, that from the 

 body and that from the lungs, to some degree separate. This degree of 

 advancement, like other beginnings, must have been slow in attainment, 



