14 UPPER EOCENE CLIMATES. 



This time of land connections was followed by sea transgression and 

 land shrinkage, especially pronounced geographically in the middle 

 Eocene (Claiborne- Jackson of the Atlantic Coastal Plain; Lutetian- 

 Auversian of the French chronology). 



During this last period, according to Osborn, the mammalia 

 undergo independent evolution without intercontinental dispersal, and 

 are sharply marked into Palaearctic and Nearctic realms, that are en- 

 tirely extinct. In Oligocene time land bridges were renewed with 

 community of terrestrial faunas, and not since that time of renewal 

 of intercontinental interchange of species have there been as free 

 water connections between the equatorial and Arctic regions as there 

 were in middle Eocene times. The geographic pattern has fluctuated, 

 to be sure, but as a whole the geography has more nearly approxi- 

 mated that of the present, and this, I believe, was the most important 

 factor in bringing about the climatic facies of the late Tertiary faunas 

 and floras. Nowhere in the North Temperate Zone, as far as I recall, 

 do we find terrestrial faunas or floras, or marine littoral faunas, in- 

 dicative of as warm conditions in the middle and upper Miocene, or 

 Pliocene, as do those of the later Eocene, and perhaps also those of 

 the early Oligocene. 



It is not impossible that some less invoked factor, such as this 

 distribution and attitude of the land and sea, may even account for 

 glacial periods, as Sir Charles Lyell suggested long ago, rather than 

 that these have been the result of causes that are purely speculative, 

 such as sun spots, carbon dioxide, dust, reversal of oceanic circulation, 

 etc. I do not deny that these latter may not have been factors, even 

 major ones, but at the present time they belong, in my judgment, in 

 the same category with that hypothesis which predicates a shifting 

 of the poles. 



I fully realize that the facts presented in the foregoing notes by 

 no means solve the difficulties that arise in our endeavors to explain 

 the distribution of organisms in the past, but that they furnish one 

 considerable factor in the uniformitarian interpretation of early Ter- 

 tiary floral distribution, is my reason for calling attention to the sub- 

 ject in the present brief way. 



Johns Hopkins University, 

 April, 1922. 



