CAUSES OF CLIMATAL CHANGE 397 



latitudes which we observe at present must have existed, prob- 

 ably in a greater degree, in the Glacial age. 



The sufficiency of the Lyellian theory to account for the 

 facts, in so far as plants are concerned, may, indeed, be 

 inferred from the course of the isothermal lines at present. 

 The south end of Greenland is on the latitude of Christiania, 

 in Norway, on the one hand, and of Fort Liard, in the Peace 

 River region, on the other ; and while Greenland is clad in 

 ice and snow, wheat and other grains, and the ordinary trees 

 of temperate climates, grow at the latter places. It is evident, 

 therefore, that only exceptionally unfavourable circumstances 

 prevent the Greenland area from still possessing a temperate 

 flora, and these unfavourable circumstances possibly tell even 

 on the localities with which we have compared it. Further, 

 the mouth of the McKenzie River is in the same latitude 

 with Disco, near which are some of the most celebrated 

 localities of fossil Cretaceous and Tertiary plants. Yet the 

 mouth of the McKenzie River enjoys a much more favourable 

 climate, and has a much more abundant flora than Disco. 

 If North Greenland were submerged, and low land reaching 

 to the south terminated at Disco, and if from any cause either 

 the cold currents of Baffin's 'Bay were arrested, or additional 

 warm water thrown into the North Atlantic by the Gulf 

 Stream, there is nothing to prevent a mean temperature of 

 45 Fahrenheit from prevailing at Disco ; and the estimate 

 ordinarily formed of the requirements of its extinct floras is 

 50, which is probably above, rather than below, the actual 

 temperature required. 



We thus know that the present distribution of land and 

 water greatly influences climate, more especially by affecting 

 that of the ocean currents and of the winds, and by the 

 different action of land as compared with water in the recep- 

 tion and radiation of heat. The present distribution of land 

 gives a large predominance to the Arctic and sub-Arctic regions, 



