of Geological Periods. 353 



warm humidity, the probable density of the atmosj)hci'c, and a 

 much less influence of latitude. It is for the stratigraphical 

 geologist to determine whether, as is generally admitted, the 

 internal heat may have still acted efficaciously in increasing the 

 temperature through the already thick crust of the rind of the 

 eartiij and whether thermal springs could rise, as they subse- 

 quently did, through beds much less folded and fractured, in 

 rocks for the most part not stratified, and in the absence of any 

 considerable elevations. Lastly, the initial temperature must 

 have undergone climatic combinations very different from those 

 of the succeeding epoch, since the eUmiiiation of most of the 

 types of this first vegetation was rapid after a certain point of 

 time, and many of them disappeared for ever. 



The temperature of the secondary periods (still consulting 

 only the indications furnished by plants) cannot have exceeded, 

 and perhaps did not even equal, that of our present intertropical 

 regions. The types of this period which still exist [Equisetum, 

 Araucaria, Encephalartos) tend to prove this. In any case the 

 climate was differently constituted, and the ground more broken 

 up than before, since the Cycadese, which now predominated, 

 are not plants of the marsh and riverside, but prefer to inhabit 

 slopes and ridges. 



The first appearance and development of the Angiospermia, 

 and especially of the Dicot3dedons, must have been the result 

 of an organic evolution ; it is impossible for us to conjec- 

 ture whether the state of the temperature contributed to it 

 at all. Great organic changes took place during the second 

 half of the Cretaceous period; and the result of these changes, 

 perhaps combined with the emergence of land, which then took 

 place on a large scale, may have been to favour the origination 

 of new types at the expense of the old ones. This movement is 

 still more strongly marked at the commencement of Tertiary 

 time, when most of the existing groups, or at least those which 

 include the ligneous plants, appear endowed with the same cha- 

 racters which still distinguish them, and which have not since 

 varied in anything essential. If the temperature seems to have 

 remained stationary, the climate, or the external conditions of 

 this temperature, appears to have changed repeatedly. Hence 

 arise very sensible variations, by the predominance or exclusion 

 of certain groups and the characteristic physiognomy of certain 

 floras. Nevertheless these exclusions could not be absolute, 

 but relative to certain regions or to the localities caj)able of fur-- 

 nishing us with impressions. The group of the Proteaccie, de- 

 veloped in the first place during the Upper Cretaceous period, 

 and effaced during the deposition of the Suessonian, reappears 

 afresh after this epoch, and presents itself as far as the Miocene. 



Ann. ^ Mag, N, Hist. Ser.3. VnL\\\. 26 



