of Geological Periods, 267 



standing the progress since made, the general result has not 

 sensibly varied. 



When compared with that of the existing world, this vegeta- 

 tion showed differences so strongly marked that one could hardly 

 help being struck by them. After having observed plants spe- 

 cifically different from those now existing, others were found 

 generically distinct, then others separated from ours even as to 

 their family, and consequently having nothing in common with 

 them but the more and more distant affinities of the class and 

 subkingdom. 



It is one of the glories of our times (and this glory belongs 

 almost entirely to M. A. Brongniart) to have collected the scat- 

 tered elements of a vegetable world so different from our own, 

 to have brought them to light and life, and to have done this 

 with such accurate ideas that they have not since been super- 

 seded. 



In endeavouring to form an idea of the temperature proper 

 to these most ancient periods of the earth^s history, we naturally 

 had recourse to the least singular plants, such as the ferns, then 

 to the Calamites regarded as Equisetacese of large size, and to 

 the Lepidodendra, arranged among the Lycopodiacese ; and it 

 appeared to follow, from the exclusive presence of plants be- 

 longing to the class of Vascular Cryptogamia, that certain 

 islands at once hot and moist, certain tropical valleys bathed in 

 tepid vapours and immersed in a dense shade, furnished a no- 

 tion of what Europe must have been like during the period 

 which corresponds with the formation of coal. 



In the ditterent composition of an atmosphere more charged 

 with carbonic acid, in the extension of the seas, in the arrange- 

 ment of the emergent land in the form of low islands, and in 

 the still sensible action of the central heat, conditions were also 

 sought which, when once admitted, accounted for the supposed 

 elevation of the temperature and for that uniformity of climate 

 which permitted the same vegetation to extend uniformly over 

 very large regions, from Spitzbergen to the East Indies, and 

 thence to Australia. Thus the preponderance and the great 

 size of the Vascular Cryptogamia and the abundance of arbores- 

 cent ferns were amongst the principal arguments invoked in 

 support of a great elevation of temperature, which was after- 

 wards explained by different causes. 



Nevertheless, without thinking of it, those who argued in 

 this way placed themselves in a vicious circle, in some respects, 

 when they invoked the influence of the central heat upon the 

 temperature to explain its elevation and the presence of arbores- 

 cent ferns in Europe, whilst these same plants appeared to other 

 geologists an evident proof of this action of the central heat. 



