1894- THE FOSSIL PLANTS OF CANADA. 179 



great earth-movements changing the geographical and cHmatal con- 

 ditions. But as the Mesozoic ages advanced, the old conditions to 

 some extent returned, and enabled the cycads, pines, and ferns of this 

 age to push their way almost to the Arctic regions. Being, however, 

 derivatives from warmer climates, their vitality and powers of varia- 

 tion were probably not great. They flourished luxuriantly and became 

 considerable coal-producers, and their reign was probably of long 

 ■duration, extending through the Triassic and Jurassic periods and 

 into the Lower Cretaceous. In the north they met with a new and 

 far more advanced and varied flora, originating there, and destined, 

 in the Middle and Upper Cretaceous, to replace them throughout the 

 whole Northern Hemisphere. This new and most important change 

 was undoubtedly accompanied with climatal amelioration, giving a 

 mean temperature of probably 55° Fahr. to regions within the Arctic 

 ■Circle ; and this, as we shall see, probably depended on geographical 

 arrangements introducing the warm waters of the equatorial current 

 into a vast land-locked basin in the interior of the American Con- 

 tinent, with corresponding though probably not so simple or easily- 

 understood arrangements in the Eastern Continent as well. 



Thus, when we ascend from the base of the Cretaceous, we find 

 a remarkable and apparently sudden influx of angiospermous exogens 

 ■ of modern generic types. The aspect of suddenness is given not 

 merely by the rapidity with which from a very few forms the new 

 flora expands in richness and variety, but also by the simultaneous 

 appearance, in the case of many genera, such a.s Sassafras, Liriodendfon, 

 Magnolia, Qiiercvs, etc., of as many species in the Middle Cretaceous 

 as the modern world can yet boast, and in some cases of a greater 

 number. On the other hand, it is true that other genera, as Populus, 

 Betula, etc., appear at first in fewer forms, and are more largely 

 represented in the modern world. This difference is apparently not 

 connected so much with the botanical rank of the several genera as 

 with their degree of adaptation to the more equable climate of the 

 earlier Cretaceous or to the more extreme climates that have 

 succeeded. This climatal change has not only required the removal 

 of some genera to southern habitats and the diminution of their 

 species, but has required that they should be replaced in the north by 

 new species of more hardy types. The southward movement of the 

 whole flora in later Cretaceous and Tertiary times, and the introduc- 

 tion from behind of the modern species, is thus apparently connected 

 with the gradual refrigeration which culminated in the Glacial 

 period. 



In Western Canada, in the Rocky Mountains, and in the Queen 

 Charlotte Islands, as well as farther north in Alaska and Greenland, 

 we have a Lower Cretaceous flora characterised b}^ forms approaching 

 those of the Jurassic. One of its characteristic species is a Dioon, 

 allied to D. edulc of Mexico, D. Columbianus and D. borcalis, Dn. 

 Along with this are species of Zamites and Podozamites, and primitive 



N 2 



