504 Evolution and Distribution of Fishes 



remained soft, plastic, and adaptable to local weightings. 

 Thus neither high mountains nor great ocean depths ex- 

 isted, but oscillations between land and water were fre- 

 quent. So wide areas now submerged were then continuous. 

 Extensive rivers, flood plains, lakes, and marshes were thus 

 formed, while diastrophic and seismic changes were fre- 

 quent and extensive. "Delta" conditions have been greatly 

 overestimated geologically and palaeontologically. 



Organisms probably evolved first during the mid Arch- 

 ean or Protobiotic stage of the earths history; while Arch- 

 ean to Lower Silurian deposits in total thickness about equal 

 the combined thickness of succeeding formations. 



The earliest organisms, as well as the ancestors of the 

 leading groups of plants and animals, all evolved in fresh 

 or inland waters, while migration seaward was a later event 

 for many. The anadromous habit therefore of sea lamp- 

 reys, sturgeons, salmon and many others is a true index to 

 the ancient environment of fishes. 



For accurate conclusions as to the evolution and en- 

 vironment of fishes exact record of every finest and chang- 

 ing rock film should be made. When such is done It appears 

 often that definite, even though thin, rock strata may ex- 

 tend over hundreds or even thousands of square miles of 

 territory. 



Some of these beds, known as "bone-beds," "oil-shales," 

 etc. are now well known from the Silurian system upward 

 to Pleistocene age. It is in the Silurian, and specially Upper 

 Silurian, system also that fish remains are first definitely 

 recognized. 



Wherever such beds, with rich remains, occur, there 

 or in close proximity to them abundant supplies of petrole- 

 um are often discovered. 



Volcanic dust showers, seismic changes, faulting and 

 folding of the earth's crust, hot-spring discharges, and other 

 agencies, caused death and entombment, often of myriads 

 of fishes, which on decay yielded the fish oil from which 

 petroleum has come. 



A comparison by Baird and Goode of the destruction 

 wrought in a season amongst Menhaden fish (3,000,000- 

 000,000,000,000), would account for more oil than is rep- 



