THE GENESIS AND MIGRATIONS OF PLANTS 215 



and finally abandoned the Arctic regions, perhaps continuing 

 to exist on the Pacific coast, and in sheltered places in the 

 north, till the warm inland seas of the Upper Cretaceous had 

 given place to the wide plains and landlocked brackish seas or 

 fresh-water lakes of the Laramie period (Eocene). Thus the 

 true Upper Cretaceous marks in the interior a cooler period 

 intervening between the Middle Cretaceous and the Lower 

 Eocene floras of Greenland. 



This latter established itself in Greenland, and probably all 

 around the Arctic circle, in the mild period of the earliest 

 Eocene, and as the climate of the northern hemisphere became 

 gradually reduced from that time till the end of the Pliocene, 

 it marched on over both continents to the southward, chased 

 behind by the modern arctic flora, and eventually by the frost 

 and snow of the Glacial age. This history may admit of cor- 

 rection in details ; but, so far as present knowledge extends, it 

 is in the main not far from the truth. 



Perhaps the first great question which it raises is that as to 

 the causes of the alternations of warm and cold climates in the 

 north, apparently demanded by the vicissitudes of the vegetable 

 kingdom. Here we may set aside the idea that in former 

 times plants were suited to endure greater cold than at present. 

 It is true that some of the fossil Greenland plants are of un- 

 known genera, and many are new species to us ; but we are 

 on the whole safe in affirming that they must have required 

 conditions similar to those necessary to their modern repre- 

 sentatives, except within such limits as we now find to hold in 

 similar cases among existing plants. Still we know that at the 

 present time many species found in the equable climate of 

 England will not live in Canada, though species to all appear- 

 ance similar in structure are natives of the latter. There is 

 also some reason to suppose that species, when new, may have 

 greater hardiness and adaptability than when in old age, and 

 verging toward extinction. In any case, these facts can account 



