PREFACE 



In 19 1 8 the writer published a volume that was entitled 

 "The Causes and Course of Organic Evolution." A section 

 of it was devoted to an inquiry into the environal relation 

 of plants and animals, as determining largely their distribu- 

 tion in time and space. The conclusion was then reached, 

 that organisms evolved first in freshwater areas, and only 

 by degrees spread into marine surroundings. 



During his inquiries the author gradually concluded 

 that fishes might form the most important biological group 

 by which to test the value of such a conclusion. For in the 

 volume named he presented short details of their past and 

 present history, which seemed to indicate that they at 

 first evolved amid freshwater surroundings. Field studies 

 also made by the writer during his earlier scientific career 

 and subsequently, strongly pointed in like direction. But 

 some details that either weakened the position, or that 

 claimed further investigation, accumulated amid the abund- 

 ant evidence that proved to be favorable. The writer there- 

 fore resolved to make as thorough a comparison as possible 

 of the groups of fishes, and even of specific genera amongst 

 these. 



The outcome is seen in part in the present volume, 

 which represents the almost continuous labors of the past 

 six years. As the author pursued his detailed researches, 

 some originally puzzling conditions gradually began to 

 have a definite meaning attached to them. Thus the pres- 

 ence of abundant fossilized fish remains in zones of cannel 

 coal, and their absence from ordinary coal strata; the like 

 occurrence of freshwater fishes and of related organisms 

 ecologically in the oil shales of the Edinburgh Coal Field 

 that the writer studied for years in his earlier career; the 

 peculiar character, the chemical composition, and the fos- 

 silized fish remains, of the marls that he studied in mature 

 years from New Jersey to Florida, all had welcome light 

 thrown on them, at the same time that the evolutionary his- 

 tory of fishes unfolded in its manifold ramifications. 



