THE EOCENE OF EUROPE AND NORTH AMERICA 95 



levels. Temperature and humidity are the most important factors govern- 

 ing plant distribution, but we must also take into consideration the nature 

 of the soil and other conditions of enviromnent. Since there are so many- 

 factors governing plant distribution, it is difficult to use plants as thermom- 

 eters of the past except in a general way, and this difficulty is increased by 

 the fact that Caenozoic species are only related to and not identical with 

 present species; also that many nearly related species can live under very 

 different conditions. 



The most memorable fact about the flora is one recently insisted upon 

 by Knowlton (1909),^ namely, that as we pass from the Cretaceous into the 

 Eocene there is no appreciable change in the flora. From this it would ap- 

 pear that there was no secular change of climate; that the temperature re- 

 mained the same. So impressed is this paltBobotanist with these facts that 

 he places within the Cretaceous the Fort Union Beds, which are here re- 

 garded as Basal Eocene. 



ALTERNATE UNION AND DISUNION OF EUROPEAN AND NORTH AMERICAN 



LIFE 



Europe and North America to-day are on the whole closely united in their 

 mammalian life, and were it not for the profound changes and extinctions 

 which have been caused by man, these widely separated countries would at 

 once be recognized as constituting one great zoological region, occupied by 

 similar forms of mammalian life. The beaver, bear, wolf, stag, moose, rein- 

 deer, bison, are some of the many connecting forms which, as Allen pointed 

 out, constitute this a single zoological region, Holarctica. 



It is a striking fact that at the beginning of Eocene times we find a similar- 

 ity which is nearly if not quite as close as that which prevails to-day. This 

 similarity of Basal Eocene times is intensified in Lower Eocene times. Then, 

 however, follows a long period of disunion in the forms and evolution of mam- 

 malian life, extending through the Middle and Upper Eocene, in course of 

 which the mammals become so different on the two continents that a zoologist 

 would certainly mark them off into two entirely distinct zoological regions, 

 namely, the Old World or Pal^arctica, and the New World or Nearctica. 



But just when the divergence seems most extreme, there comes at the 

 beginning of the Oligocene a fresh faunal reunion, perhaps even more close 

 than the first. These periods of union and separation again recur. We thus 

 have good ground for dividing the whole Caenozoic Period into a series of 

 grand Faunal Phases. 



a. Faunal phases. — Another means of distinguishing these faunal 

 phases, in addition to the continental separation and reunion of the manmials, 



' Knowlton, F. H., The Stratigraphic Rolations and Palaeontology of the "Hell Creek 

 Beds," "Ceratops Beds" and Equivalents, and their Reference to the Fort Union Forma- 

 tion. Proc. Wash. Acad. Sci., Vol. XI, no. 3, 1909, pp. 179-238. 



