SUN AND CLIMATIC VARIATION 51 



extending from lat. 50 to lat. 71 , however, was a temperate 

 one, and annual alternations of temperature are indicated in 

 the trunks of certain conifers, notably in those discovered in 

 Graham's Land on the edge of the Vancouver Straits, and in 

 the Araucaris by the concentric rings known in the conifers 

 and dicotyledonous trees of our country, but which are absent 

 in trees of the torrid zone. Thus there were evidently seasons 

 in the polar regions, which, however, continued to enjoy 

 a very mild climate throughout the Jurassic Period, whereas a 

 tropical climate persisted in what are the present temperate 

 regions. For if the corals disappeared suddenly in the north of 

 the Central Plateau, doubtless as the result of a change in the 

 direction of the currents, they appear again in the Tethys, 

 beginning at Poitou, and also, somewhat later, in the 

 Ardennes, and still later near Trouville, on the eastern frontier 

 of Lorraine, to the north of Morvan, at Bourges, Sancerre, 

 and even extend to Yorkshire ; while in the Jura they lasted 

 still later. 



These conditions changed very quickly in the Cretaceous 

 Period which followed. The corals were entirely replaced in 

 the Mediterranean area by the Rudistae, peculiar lamelli- 

 branch molluscs, which also congregated in immense reefs 

 and came from warm waters, though they accommodated 

 themselves to a lower temperature. Caducous dicotyledons 

 made their appearance and developed more and more in the 

 northern regions. The temperature, however, continued, if 

 not very high, at least mild and fairly constant as it is to-day 

 on the coasts of Brittany, for the bread-fruit tree and various 

 cycads flourished side by side with small-flowered dicotyledons, 

 such as Willows, Poplars, Birches, Oaks, Walnuts, Plane-trees, 

 and Figs, and evergreens like Ivy and Oleander. Some 

 gamopetalous forms — the Viburnum, and even mono- 

 cotyledons were associated with them. 



During the Nummulitic Period, which marks the beginning of 

 the Tertiary epoch, Greenland and Spitzbergen still retained 

 a very rich flora, and even kept it through the Neogene Period 

 that followed, which clearly demonstrates that the polar 

 regions had not as yet experienced any considerable decrease 

 in temperature. The temperature indeed was almost that of 

 the present Mediterranean lands. Grinnel Land, in lat. 82 , 

 had the climate the Vosges have to-day ; poplars, birches, 



