432 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



After supplemental oxygenation by air-breathing had become established 

 as a necessity of the climate, the possibility arose of continued activity 

 during the season of shrinking waters, and this required that the fringed 

 limb, suited for subaqueous crawling, as well as guidance in swimming, 

 should now become modified and strengthened for subaerial crawling, in- 

 volving, as it does, the partial support of the body, but losing very largely 

 its original swimming function. 



POSSIBLE CONSEQUENCES OF ANOTHER MODE OF SUPPLEMENTAL 



RESPIRATION 



In regard to the mode of supplemental use of air by means of an air- 

 bladder which became established among the early ganoid fishes, the fact 

 that this was but one of several possible modes is shown among certain 

 modern fishes by the actual employment of various other devices. 

 Slightly different habits in meeting slightly different conditions, adventi- 

 tious and inconsequential, so far as the immediate needs were concerned, 

 would have directed the Silurian-Devonian fishes into other modes of 

 using air and might have readily shunted onto another track the whole 

 subsequent course of vertebrate evolution. 



VENT AUK , 



Figure 2. — Diagrams illustrating a possible Mode of Evolution of Air-breathing 



A. Accessory breathing organs as they might have been developed in the gill chamber. 

 B. Resultant simplified circulation of air-breathers which would have resulted. 



In figure 2 is shown what might have been the result if the first fishes 

 which began to use air had developed within the gill chamber, in connec- 

 tion with several gill arches, labyrinthiform or arborescent growths for 

 the utilization of air. No alteration of the heart and circulatory system 

 would have been required. A mere increase in size of the respiratory 

 organs and of strength in the heart would have provided the mechanism 

 for Avarm-bloodedness and more sustained bodily activity. Contrast this 

 possibility of rapid evolution with the long geologic ages which passed 



