390 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



verging lines of evidence that the rise of amphibians came not from the 

 sea, but from the land waters. The exposure of the tidal zone alternately 

 to water and to air had, then, nothing to do with the origin of lungs. 



Having made this study of environments, the argument passes on to 

 an analysis of causes leading to the rise of amphibians. The law of 

 probabilities shows that the directive influence of external factors is 

 necessary to guide the development of old organic structures into com- 

 binations of new structures which shall be efficient under a combination 

 of new conditions. Natural selection, although not now regarded as an 

 explanation of most minor organic variations and the development of 

 new species, is nevertheless a broad controlling force which compels de- 

 velopment within certain limits of efficiency. What, then, were the 

 causes which controlled the passage of fishes into amphibians? The 

 chief cause is found to have been the nature of Silurian and Devonian 

 climates. The warm and stagnant waters of the dry season compelled 

 those fishes which should survive to make larger and larger use of air. 

 The organic nature of fishes was at that time happily able to vary in 

 pace with the demands of changing environment. 



The evidence is regarded as strong that the air-bladder was originally 

 developed as a supplemental breathing organ, although in modern fishes 

 it has been mostly diverted, to other uses. Among certain Devonian 

 fishes, living under more and more strenuous climatic conditions of 

 seasonal dryness, the use of the air-bladder for respiration became essen- 

 tial, and with the diminishing availability of the waters of certain regions 

 the gills in those species which survived this crisis in evolution became 

 correspondingly atrophied. The amphibians thus arose under the com- 

 pulsion of seasonal dryness. 



In conclusion, it is noted how the particular method of accessory 

 respiration which was adopted by the ancestors of amphibians was only 

 one of several methods which have been used by fishes. This method of 

 accessory respiration permitted the rise of land vertebrates and deter- 

 mined the future lines of evolution, but another choice of the mode of 

 respiration might have led to more rapid progress — a progress which 

 would, however, have been directed into somewhat different lines. 



Lastly, the rise of amphibians from river fishes in an epoch of semi- 

 aridity was one of the major steps in the evolution of man. Matthew 

 has developed the evidence for the hypothesis that "the evolution of land 

 life in adaptation to recurrent periods of aridity supplies a satisfactory 

 background of cause for the whole evolution of the higher vertebrates." 3 



3 W. D. Matthew : Climate and evolution. Annals of the New York Academy of Sci- 

 ences, vol. xxiv, 1915, pp. 171-31S (p.. 181). 



