96 Evolution and Distribution of Fishes 



CHAPTER IV. 



The Physical and Biological Environment of Fishes. 

 (a) During the Silurian and Devonian Epochs. 



It may be accepted as an almost invariable principle 

 that a study of the environal relations of any group of 

 organisms will prove a highly helpful guide in determining 

 the exact lines of evolutionary progression pursued by 

 such organisms. We desire now to apply such a principle 

 to fishes. If, as already emphasized in the previous chap- 

 ter, they developed from freshwater Nemerteans, we might 

 expect to find that so soon as any method of fossilization, 

 or any organismal parts capable of being fossilized, had 

 developed, traces might be got of the organisms of the 

 period in question. If, moreover, several distinct groups 

 of plants and of animals are found to be preserved side 

 by side in the same strata, such would pretty surely indicate 

 that these were more or less closely associated during life, 

 if not in the same environal medium, at least in one re- 

 moved at no great distance. 



A study of the mass of stratified rocks that makes up 

 the Laurentian and Huronian systems of America has failed 

 to reveal recognizable organic remains, except possibly in 

 the upper strata of the Huronian. We are compelled to 

 accept it, however, that a varied invertebrate fauna had 

 already appeared, though most or all of the existing organi- 

 isms must have been soft-bodied. But when the Cambrian 

 rocks are reached an abundant marine invertebrate fauna is 

 encountered, the literature on which is synopsized in the 

 textbooks of Chamberlin-Salisbury (5:276-303), of Prest- 

 wich, of Geikie (7: II: 933-941) and others. Thus grapto- 

 lites, corals, trilobites, brachiopods, cephalopods and other 

 molluscs are often abundant. These form a biological 

 congeries that at once stamps the rocks in which they 

 occur as of marine origin. So far as yet revealed no 

 traces of lacustrine, fluviatile or land organisms have been 

 noted But land undoubtedly existed; for cliffs of erosion, 

 strata composed of fine shales or limestones, of coarser 

 mudstone or sandstone, and even rough conglomerates, are 

 all encountered often in continuous series. The writer has 



