THE GENESIS AND MIGRATIONS OF PLANTS 21/ 



vast areas, both on the margins and in the interior of the 

 continent, were occupied with swampy flats and lagoons, the 

 atmosphere of which must have been loaded with vapour, and 

 rich in compounds of carbon, though the temperature may 

 have been lower than in the Devonian. There still remained, 

 however, more especially in the west, a remnant of the old 

 inland sea, which must have greatly aided in carrying a warm 

 temperature to the north. 



If now we pass to the succeeding Jurassic age, we find a 

 more meagre and less widely distributed flora, corresponding 

 to less favourable geographical and climatal conditions, while 

 in the Cretaceous and Eocene ages a return to the old con- 

 dition of a warm Mediterranean in continuation of the Gulf of 

 Mexico gave those facilities for vegetable growth, which 

 carried plants of the temperate zone as far north as Greenland. 



It thus appears that those changes of physical geography 

 and of the ocean currents to which reference is so often made 

 in these papers, apply to the question of the distribution of 

 plants in geological time. 



These same causes may help us to deal with the peculiarities 

 of the great Glacial age, which may have been rendered ex- 

 ceptionally severe by the combination of several of the conti- 

 nental and oceanic causes of refrigeration. We must not 

 imagine, however, that the views of those extreme glacialists, 

 who suppose continental ice caps reaching half way to the 

 equator, are borne out by facts. In truth, the ice accumulat- 

 ing round the pole must have been surrounded by water, and 

 there must have been tree-clad islands in the midst of the icy 

 seas, even in the time of greatest refrigeration. This is proved 

 by the fact that in the lower Leda clay of Eastern Canada, 

 which belongs to the time of greatest submergence, and whose 

 fossil shells show sea water almost at the freezing point, there 

 are leaves of poplars and other plants which must have been 

 drifted from neighbouring shores. Similar remains occur in 



