40 PROCEEDINGS OF THE WASHINGTON MEETING 



Extensive volcanic material is intermixed. The present writer interprets them 

 as dominantly fluviatile deposits, formed under semi-arid climatic conditions. 

 This stands between the extreme interpretations, but is essentially different 

 from either. 



The subject is important from the standpoint of general and historical 

 geology, but perhaps especially from that of organic evolution, since these 

 deposits contain the oldest well known fish faunas in contrast to the rare and 

 fragmentary fossils of the previous periods. If the stratigraphic record is 

 misinterpreted, there is given, in consequence, an erroneous conception of the 

 habitat and life adaptations of the advancing evolutionary wave of the verte 

 brate phylum when it first clearly appears in the geologic record. 



A reinterpretatioii must rest on the criteria as to the mode of origin of 

 sediments as much as on the actual stratigraphic features. It is the matching 

 of more recent criteria to the older known facts of the stratigraphy of the 

 old red sandstone which makes the basis of this paper. 



Presented in abstract extemporaneously. 



Eemarks were made by Messrs. A. W. Grabau and J. M. Clarke, with 

 reply by the author. 



INFLUENCE OF SILURIAN-DEVONIAN CLIMATES ON THE RISE OF AIR- 

 BREATHING VERTEBRATES 



BY JOSEPH BARBELL 



(Abstract) 



The relationships of ancient faunas to their environments is a field wherein 

 paleontology and physical geology meet. It is a field which has been commonly 

 cultivated by the former, but it is one in which the latter may as logically 

 enter. It was as a physical geologist, with ideas sharpened and made definite 

 by previous study of the nature of Devonian sediments, that the present writer 

 took up this subject. 



The Devonian formations indicate the general presence of warmth and 

 seasonal rainfall. In the Upper Devonian the general climatic conditions be- 

 came more markedly semi-arid. There is found to be a concurrent elimination 

 of sharks from the fresh waters. As a result, dipnoans and crossopterygians 

 come to dominate the fauna. It is apparently in the Upper Devonian, further- 

 more, that amphibians began to expand. An examination is made of the 

 various possible causes for this advance in evolution. The only one which is 

 found adequate is the compulsion of seasonal dryness. 



The actual line along which air-breathing developed was only one of several 

 possible lines. The directions and limitations of later evolution were, how- 

 ever, more or less determined by this choice. Slightly different conditions of 

 environment and organic response, both readily possible, might apparently 

 have been more favorable for the future of air-breathing vertebrates. By 

 appreciating these lost opportunities of the remote past a better perspective is 

 obtained, on the one hand, of the devious and groping and not always most 

 successful nature of organic progress ; a better appreciation, on the other hand, 

 of the dangers of stagnation or extinction which have been happily passed by. 



This and the previous paper were presented by the writer in abstract to the 



