THE GENESIS AND MIGRATIONS OF PLANTS 213 



characteristic plants, have been traced into the British territory 

 north of the forty-ninth parallel, and it has been shown that 

 their fossils are identical with those of the McKenzie River 

 Valley, described by Heer as Miocene, and probably also with 

 those of Alaska, referred to the same age. 1 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 me to doubt the Miocene age of the upper 

 plant-bearing beds of Greenland, and more recently Mr. J. 

 Starkie Gardner has shown from comparison with the Eocene 

 flora of England and other considerations, that they are really 

 of that earlier date. 2 



In looking at these details, we might perhaps suppose that 

 no conditions of climate could permit the vegetation of the 

 neighbourhood of Disco in Greenland to be 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 tem- 

 perate Europe to those of Greenland. If Greenland required 

 a temperature of about 50, as Heer calculates, to maintain its 

 " Miocene " flora, the temperature of England must have been 

 at least 70, and that of the south-western States still warmer. 

 It is to be observed, however, that the geographical arrange- 



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

 1875, where full details on these points may be found. 



2 Nature, Dec. I2th, 1878 ; Publications Palceontographical Society ; 

 Reports to British Association. It seems certain that the so-called Miocene 

 of Bovey Tracey in Devon, and of Mull in Scotland, is really Eocene. The 

 Tertiary plant-bearing beds of Greenland are said by Nathorst to rest un- 

 conformably on the Cretaceous, and are characterized by M'Clintockia and 

 other forms known in the Eocene of Great Britain and Ireland. 



