THE REVOLUTIONS OF THE CRUST OF THE EARTH. 323 



and Spitzbergen, regions now permanently covered with ice, are evidence 

 of a uniform climate very favorable to the increase of vegetation. For 

 a long time it was matter of discussion how trees could flourish and 

 form forests so near the pole, since the solar light failed them during a 

 large part of the year, and the heat was insufficient. It is, however, 

 not difficult to conceive that plants do not require light when, through 

 extreme cold, which deprives them of their leaves, they go to sleep, so 

 to say, in order to await the return of light and heat. It would be only 

 necessary for their preservation that the cold to which they were ex- 

 posed was not too extreme.* Now the heat which mitigates the rigor 

 of the climate is furnished by the oceanic currents, proceeding from the 

 equatorial regions. 



According to Humboldt, currents depend upon the tides in their move- 

 ment around the globe, upon the duration and force of dominant winds, 

 upon the difference of the specific gravity of the waters or their degree of 

 saltness, and finally upon the horary variations of the barometric press- 

 ure. Now, all these causes which influence the production of ocean cur- 

 rents are variable. The movement of the tides is modified by the config- 

 uration of the continents ; the winds by the unequal distribution of the 

 land and water, the very different calorific capacity of which produces 

 ascending currents, the real generators of aerial currents. The difference 

 in saltness is produced by the great quantity of fresh water which is 

 emptied into the ocean, or by the abundance of the rain, very variable 

 upon the sea. The result is that the currents themselves are variable : 

 and if to-day the western coasts of Europe are visited by the Gulf Stream, 

 in former times this current may have been directed more to the north, 

 and promoted upon the American islands and upon Spitzbergen the de- 

 velopment of vegetation. The cold currents, on the contrary, may have 

 proceeded along the European coasts and produced there a climate simi- 

 lar to that of Canada and Labrador. 



The climatic conditions may also be changed by the predominant 

 distributions of the sea upon a hemisphere, and in this case a damp and 

 foggy climate would be the consequence! Now, such a climate, even 

 were it quite cold, would not prevent the development of a rich crypto- 

 gamic vegetation, as is shown by the flora of the islands of Falkland 

 and of New Zealand. There is nothing, then, averse to the supposition 

 that, at certain geological epochs, the climatic conditions were very 

 favorable to the development of plants, and that their growth was 

 possible even in the polar regions. Moreover, the coal-beds of the isl- 

 ands of Disco and Spitzbergen cannot bo the result of the accumulation 

 of floating wood, for, in the first place, the debris thus deposited would 

 be mingled with sand and pebbles, and, in the second, trunks of trees 

 still standing have been found in these beds. 



Plants act a very important part in the economy of nature. They 

 are the immediate agents of organization, and serve as intermediates 



# J. Croll, On Geological Time, etc., Philos. Magaz., vol. sxxv, 1868, p. 445. 



