388 J. BARRELL INFLUENCE OF CLIMATES ON VERTEBRATES 



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Relations of habit in primitive fishes to the development of paired fins. 430 

 Possible consequences of another mode of supplemental respiration . . . 432 

 Conclusion on the dangers which have threatened organic progress . . 434 



Introduction and Summary 



The problems of organic evolution have many aspects and ramify into 

 many fields of science. The subject was at first embraced chiefly in the 

 field of the old-time naturalist — zoologist or botanist — but the problems 

 of variation and heredity have passed into the hands of the experimental 

 evolutionist; and there are other problems whose answers are found in 

 the geologic record, but these are of two rather opposite aspects. On the 

 one hand, the paleontologist specializes particularly on the succession 

 and relation of fossil faunas or floras. On the other hand, it is the field 

 of physical and historical geology to restore the ancient environments. 

 The relations of the environments to the biotas is a field wherein physical 

 geology and paleontology meet, to give a better understanding of the 

 underlying causes of organic response and progress. It is from the 

 standpoint of physical and historical geology rather than from that of 

 paleontology that the present study is made. 



A summary of the paper is as follows : 



It is shown to be probable that fishes arose in land waters. As such 

 they constituted primarily a river fauna. The fossil record, as known 

 at present, shows that about the middle of Ordovician time fragments of 

 armored fishes were occasionally drifted out to sea and worn by waves 

 and currents. Some of these lowly fishes came later to live in brackish 

 waters and are found in bay or marginal marine deposits. They clearly 

 did not as yet rule the sea, nor could they then withstand the enemies 

 which lived therein. In the later Silurian strata the spines of primitive 

 sharks and plates of ostracoderms begin to be found more abundantly, 

 but only in a few localities and in special beds. Their life was still 

 limited to brackish and protected waters ; their normal home, serving as 

 a center of dispersal, was apparently still within the land waters. Not 

 until the beginning of the Middle Devonian, as powerful sharks and 

 arthrodires, did the fishes first really begin to conquer the ocean and its 

 former rulers. Then they began also, for the first time, to leave abundant 

 and well preserved fossils in marine deposits unrelated to the shores. 2 



2 Professor Grabau has quite independently reached very similar conclusions in regard 

 to the geological history of ostracoderms and fishes. These are expressed in his "Prin- 

 cipals of stratigraphy," pp. 1033-1035, 1913. Since then Grabau has transmitted a 

 manuscript on The Devonic fish fauna of America, to the Michigan Geological Survey. 

 This is still unpublished. In it he discusses in detail the problem of the habitat of the 



