232 Evolution and Distribution of Fishes 



the lapse of "a vast period" during which slow variation 

 as well as obliteration were effected. While we would by 

 no means minimize the need for extended geologic time 

 the evidences rather are that enormous and rapid move- 

 ments of the earth's crust at once extinguished great cohorts 

 of organisms, and also caused the relatively few that were 

 left to vary rapidly and strikingly, as they passed into 

 new environal territory; or as they often became cut off 

 from continuity-association with other individuals of their 

 species; and as they then became adapted by proenvironal 

 response to their new surroundings. 



When it is fully recognized that such changes did occur, 

 and that the ocean was gradually stocked with its groups 

 of marine fishes, by steady migration of these out of fresh- 

 water areas into the sea, and from late Jurassic time till 

 now, problems that otherwise seem confused and even in- 

 explicable, become relatively simple, as later pages will 

 explain. 



With the progressive upheaval of mountain masses, 

 from late Cretaceous to the close of Miocene times, that 

 greatly excelled in height and bulk any that had previously 

 been formed over the earth's crust, greatly accelerated 

 denudation changes must have progressed over dry lands, 

 especially over those in proximity to, and sloping rather 

 abruptly from, the mountain ranges. This also would favor 

 the expanse and damming up of river courses, the conse- 

 quent formation of extensive lakes, the origin of changed 

 environal states, and resulting modification of the denizens 

 of the lakes and rivers. The great tracts that were covered 

 by lakewaters in N. W. America, in East Brazil, in central 

 Africa, in central and west Europe, in Tasmania and other 

 lands are proofs that such took place. 



I. Eocene Formation. Restricting attention first to 

 the Eocene formation, extensive freshwater and marine 

 deposits are now known that enable us to form a fair idea 

 of general environal relations. The at times gradual, at 

 times sudden, change from the one type of deposit to the 

 other, tells that slow or rapid elevation or depression of 

 the land was proceeding in many regions, some of which 

 are touched on below. Whether such was due to volcanic 



