GENERAL LAWS OF ORIGIN AND MIGRATION. 251 



took its origin in the north and advanced southward over 

 ney lands in process of emergence from the sea. Tho 

 somewhat similar condition evidenced by the Lower Car- 

 boniferous limestone preceded the advent of the great and 

 lich flora of the coal-formation. 



Lyell's theory on this subject has, I think, in some re- 

 cent publications, been somewhat misapprehended. It 

 is true that he stated hypothetically two contrasted con- 

 ditions of distribution, in one of which all the land was 

 equatorial, in another all polar ; but he did not suppose 

 that these conditions had actually occurred ; and even in 

 his earlier editions, before the recent discoveries and dis- 

 cussions as to ocean currents, he was always careful to at- 

 tach due value to these in connection with subsidences 

 and elevations.* In his later editions he introduced 

 more full references to current action, and also stated 

 Croll's theory, but still maintained the validity of his 

 original conclusions. 



The sufficiency of this Lyellian theory to account for 

 the facts, in so far as plants are concerned, may, I think, 

 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. f It is evident, therefore, that only excep- 

 tionally unfavourable circumstances prevent the Greenland 

 area from still possessing a temperate flora, and these un- 

 favourable circumstances possibly tell even on the locali- 

 ties with which we have compared it. Further, the 

 mouth of the McKenzie River is in the same latitude with 



* See " Principles of Geolofry," edition of 1840, chapter vii. 

 f See " Macoun's Report," " Geological Survey of Canada," and Rich- 

 ardson's " Boat Voyage." 



