428 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



but it had to be reached in those ganoid fishes which, by virtue of their 

 power to use air, have been able to occupy a special habitat and survive 

 through the long ages which have elapsed since the Devonian. From this 

 stage forward the existing relics froni the past show the slow progress of 

 that profound and fundamental further respiratory-circulatory transfor- 

 mation which has been necessary to permit the advancement of verte- 

 brates from the lowly amphibian to the active and intelligent mammal. 



COMPULSION OF SEASONAL DRYNESS 



Thus an intestinal method of using air for supplemental respiration 

 which initially had no clear advantage over another possible method, that 

 by means of the pharyngeal chamber, but later had a disadvantage, was 

 assumed through fortuitous choice and fixed by habit. It represented a 

 parting of the ways. This blind choice of the ganoid fishes, directed by 

 minor and unessential factors, fixed for all time the lines of evolution of 

 vertebrates in regard to the fundamental life activities of respiration and 

 circulation. 



If lungs had been developed in the pharvngeal chamber and the use of 

 air had thus involved no general upsetting of the circulatory system, then, 

 irrespective of the compulsion of seasonal dryness, it is conceivable that 

 a transition might readily have occurred to a larger use of air. Such a 

 transition would have involved an increase of efficiency throughout the 

 change. Once started it could have been carried forward merely from 

 such an evolutionary momentum as has been called orthogenesis. 



With that method of air-breathing which was adopted, however, the 

 growing disuse of the gills resulted in a decrease instead of an increase 

 in the efficiency of the organism. What had served very well as a sup- 

 plemental organ became very inefficient as a substitute organ. To adapt 

 it to sole use and still permit the animal to live, profound changes be- 

 came necessary in the heart and its connections with the lungs. This 

 remaking of the circulatory system as a result of the throwing of the 

 burden of respiration on the rudimentary lungs is a measure of the com- 

 pulsion of nature. The fish which had to make larger use of air instead 

 of water was not as well off as the fish which in another region could still 

 breathe only water, provided that the water was well aerated. Such a 

 makeshift apparently could have been possible only where aerated water 

 was not available in the environment. If. further, the fishes had been 

 able to retreat from the seasonally untoward conditions, this would have 

 been easier than the organic transformation. But if the fishes were 

 trapped in rivers which in the dry season shrunk to crowded pools and 

 in lakes which recurrentlv were reduced to slimv mud flats or even dried 



