GENERAL LAWS OF ORIGIN AND MIGRATION. 245 



McKenzie Eiver valley, described by Heer as Miocene, 

 and probably also with those of Alaska, referred to the 

 same age.* Now this truly Eocene flora of the temperate 

 and northern parts of America has so many species in 

 common with that called Miocene in Greenland that its 

 identity can scarcely be doubted. These facts have led 

 to scepticism as to the Miocene age of the upper plant- 

 bearing beds of Greenland, and more especially Mr. J. 

 Starkie Gardner has ably argued, from comparison with 

 the Eocene flora of England and other considerations, 

 that they are really of that earlier date, f 



In looking at this question, we may fairly assume that 

 no climate, however equable, could permit the vegeta- 

 tion of the neighbourhood of Disco in Greenland to be 

 exactly identical with that of Colorado and Missouri, at a 

 time when little difference of level existed in the two 

 regions. Either the southern flora migrated north in 

 consequence of a greater amelioration of climate, or the 

 northern flora moved southward as the climate became 

 colder. The same argument, as Gardner has ably shown, 

 applies to the similarity of the Tertiary plants of temper- 

 ate Europe to those of Greenland. If Greenland required 

 a temperature of about 50°, as Heer calculates, to main- 

 tain its Eocene flora, the temperature of England and 

 that of the Southwestern States must have been higher, 

 though probably more equable, than at present. 



"We cannot certainly affirm anything respecting the 

 migrations of these floras, but there are some probabilities 

 which deserve attention. The ferns and cycads of the 

 so-called Lower Cretaceous of Greenland are nothing but 

 a continuation of the previous Jurassic flora. Now this 

 was established at an equally early date in the Queen 



* G. M. Dawson, " Report on the Geology of the Forty-ninth Parallel," 

 where full details on these points may be found. " Transactions of the 

 Royal Society of Canada," vol. iv. 



f "Nature," December 12, 1878. 



