THE TERTIAKY TO THE MODERN PERIOD. 221 



ferent latitudes. However equable the climate, there 

 must have been some appreciable difference in proceed- 

 ing from north to south. If, therefore, as seems in 

 every, way probable, the new species of plants origi- 

 nated on the Arctic land and spread themselves south- 

 ward, this latter process would occur most naturally in 

 times of gradual refrigeration or of the access of a 

 more extreme climate — that is, in times of the elevation 

 of land in the temperate latitudes, or, conversely, of 

 local depression of land in the Arctic, leading to invasions 

 of northern ice. Hence, the times of the prevalence of 

 particular types of plants in the far north would precede 

 those of their extension to the south, and a flora found 

 fossil in Greenland might be supposed to be somewhat 

 older than a similar flora when found farther south. It 

 would seem, however, that the time required for the ex- 

 tension of a new flora to its extreme geographical limit is 

 so small, in comparison with the duration of an entire 

 geological period, that, practically, this difference is of 

 little moment, or at least does not amount to antedating 

 the Arctic flora of a particular type by a whole period, 

 but only by a fraction of such period. 



It does not appear that, during the whole of the Cre- 

 taceous and Eocene periods, there is any evidence of such 

 refrigeration as seriously to interfere with the flora, but 

 perhaps the times of most considerable warmth are those 

 of the Dunvegan group in the Middle Cretaceous, and 

 those of the later Laramie and oldest Eocene. 



It would appear that no cause for the mild tempera- 

 ture of the Cretaceous needs to be invoked, other than 

 those mutations of land and water which the geological 

 deposits themselves indicate. A condition, for example, 

 of the Atlantic basin in which the high land of Greenland 

 should be reduced in elevation, and at the same time the 

 northern inlets of the Atlantic closed against the invasion 

 of Arctic ice, would at once restore climatic conditions 



