From Eocene to Recent Time 231 



CHAPTER VIII. 



The Physical and Biological Environment of Fishes. 

 Eocene to Recent. 



In beginning our study of the relation of fishes to en- 

 vironal conditions during deposition of the Tertiary rocks, 

 a fundamental fact to be again remembered is the steady 

 or rapid alteration in land and water areas that resulted 

 in new geographical configurations. These culminated dur- 

 ing late Pliocene and Pleistocene time in the main conti- 

 nental masses and outlines that we now see. Such changes 

 were due to an increased soldification and thickening of the 

 earth's crust; to the extrusion of those enormous igneous 

 beds in India, Africa, America and to a less degree in other 

 regions, that were referred to in last chapter; and to the 

 resulting upheaval during Eocene to late Miocene time of 

 the most elevated mountain chains that now exist. As a 

 coeval result and in large measure also as a result of exten- 

 sive earth shrinkage or diastrophism, great cracks, faults, 

 downthrows and upthrusts, as well as foldings, caused a 

 breaking up of previous continuous land-masses over the 

 South Atlantis, the North Atlantis, "Lemuria" and other 

 regions, which in turn started profound organic change and 

 destruction as well as evolution. 



While it is true that over wide areas in North America, 

 in south-east Europe and elsewhere, there is a gradual 

 merging and conformability of upper Cretaceous strata, 

 as they pass into the lowermost Eocene beds above, it is 

 more usual to find that marked alterations and uncon- 

 formabilities occur. And this is in line with the fact that a 

 tremendous and rapid obliteration of organisms — not least 

 of fishes — took place during transition from Senonian and 

 Danian, in other ^ords from Upper Cretaceous, to lower 

 Eocene beds. So, as Woodward's descriptions show, very 

 few genera of freshwater Cretaceous fishes are continued di- 

 rectly into Eocene. The number of marine genera however 

 is somewhat greater. By some writers it has been claimed 

 that the fundamental change seen in organic life indicated 



