THE MIOCENE PERIOD. 283 



grasses, which in turn influenced the evolution of the mammals in the 

 lines already pointed out. 



How far the gradual removal to the south of the forms now regarded 

 as tropical or subtropical, and the concentration at the north of the 

 forms that now characterize those latitudes, was the result of a natural 

 differentiation and segregation of the previously mixed forms, and 

 how far the result of a progressive differentiation of climate, it is per- 

 haps unsafe to say; it has usually been attributed to the latter. It 

 has been customary to interpret the climatic implications of the Ter- 

 tiary floras by the southern forms, such as the palms, magnolias, figs, 

 etc., and to ignore the northern forms, poplars, willows, etc. For 

 this there are apparently some good reasons, but it is not clear that 

 they are conclusive. 



According to Heer, 1 there were Miocene forests in high latitudes 

 (Nova Zembla, Spitzbergen, Iceland, Greenland, Grinnell Land, Banks 

 Land, the mouth of the Mackenzie, and Alaska) which contained pines, 

 cypresses, birches, maples, walnuts, poplars, elms, oaks, lindens, wil- 

 lows, hazels, and even magnolias and tulip-trees. Question has how- 

 ever been raised as to the period to which these belong, and as the areas 

 are all isolated, stratigraphical tracing is impracticable. It seems 

 not impossible that they were Eocene. When, as in a case like 

 this, there is ground to suspect that faunas and floras are forced by 

 climatic changes to migrate rather rapidly in latitude, the basis of 

 correlation by fossils is disturbed, for the existence of the same faunas 

 and floras in different latitudes does not prove contemporaneity; it 

 may only mean successive occupancy by forced migration. Exact 

 correlations therefore become very difficult. But the occurrence of 

 these plants in so high latitudes in either the Eocene or Miocene is 

 sufficiently remarkable. 



The Land Animals. 



The earlier fauna. — The early Miocene of North America (John 

 Day epoch) was separated by a long interval from the late Miocene 

 (Loup Fork epoch) and this gave a marked distinctness to the faunas 

 of the two epochs. The earlier resembled the Oligocene (White River) 

 fauna in general aspect, but most of the mammalian genera, and nearly 



1 Flora Fossilis Arctica, Vol. I, p. 161. 



