Related Geological Conditions 33 



So it may well be inferred that, over wide continental 

 areas both of the northern and southern hemispheres, ex- 

 tensive river systems drained wide regions of comparatively 

 low land, and that in their course they may have formed 

 huge marshy lakes of varying depth but temporary duration. 

 When these flood-plains or swollen lakes dried up masses 

 of fish may have become stranded, buried under the finer 

 mud deposits and then dried under hot suns, thereafter to 

 undergo fossilization in at least some cases. 



IV. The geologic age of continents. 



Another widely discussed and greatly disputed problem 

 in geology has been the relative permanence of existing 

 continental masses, as compared with the possible former 

 distribution of land and sea in a totally different manner. 

 From the biological standpoint, and not least from that of 

 the history and dispersal of fishes, a fundamentally different 

 disposal of the great land'areas from that of the present or 

 of recent time, is a prime necessity that will become evident 

 as we proceed. So the outline charts suggested by Neumayr, 

 Schuchert, de Lapparent, Freeh and others, as being ex- 

 planatory of past land and sea areas, have much in their 

 favor, and especially so in connection with the gradual 

 evolution and distribution of fishes. Though only approxi- 

 mate in exactness probably, they are of extreme value, and 

 will be reproduced in unaltered or in modified form, as 

 occasion requires in the succeeding text. 



The hitherto prevalent view that fishes as a group origi- 

 nated in the sea, could most easily be supported in a super- 

 ficial manner on the hypothesis that the existing oceans are 

 fundamental and are ancient reservoirs of salt water, from 

 which successive races of fish could pass inward to the rivers 

 and lakes of encircling land masses. But geologists and 

 zoologists have alike failed to realize, that those genera 

 which show at present the most marked tendency to pass 

 from a saline to a fresh-water environment, belong to larger 

 aggregates which are mainly fresh-water inhabitants. Thus 

 the lampreys, the sturgeons, the salmon, the shad and many 

 other anadromous fishes have had — as we shall later show 

 — a freshwater origin and history up till now. 



