the Earth at different Epochs^ <J-c. 9ffi 



difficult not to admit, that at this period the temperature of the 

 earth''s surface was equal, and perhaps superior, to tliat of the 

 warmest parts of our globe. This inference, moreover, is in 

 perfect accordance with the opinions of most of our present 

 geologists, and widi the observations and theories of the most 

 celebrated natural philosophers and mathematicians, 

 v The second remarkable character of the flora of this period 

 consists in the numerical relation of the plants of the different 

 classes, and which is such that the vascular cryptogamia, which 

 at the present epoch form at the most one- tenth of the plants 

 known, then constituted more than nine-tenths of the vegetation. 



There does not, therefore, exist the smallest relation between 

 the primitive flora of our globe and the mass of its present ve- 

 getation. But are there not some parts of the globe, some par- 

 ticular regions, which, without presenting exactly the same pro- 

 portions between the different classes of the vegetable kingdom, 

 at least approach in this respect to the ancient flora ? 



This, in fact, is what the study of the distribution of vegeta- 

 ble forms over the surface of the fglobe demonstrates to us. 

 The family of Ferns, and the allied families, according to the 

 observations of Mr R. Brown and M. d'Urville, appear to be 

 subjected to the influence of two principal causes, which deter- 

 mine their mode of distribution. 



Of these two causes, one is the elevation of the temperature ; 

 the other appears to be the influence of the humid air, and uni- 

 form temperature of the sea. 



There results from these causes, that, in the localities equally 

 favoured by them, these plants are more frequent in the equa- 

 torial zone than in the colder zones ; but that, in the same zone, 

 they are much more abundant in the islands than upon the con- 

 tinents. We might adduce numerous examples in support of 

 this proposition, but this would lead us too far from the parti- 

 cular subject of the memoir, and we shall only say, that, in the 

 parts most favourable to the development of these plants on the 

 continent of temperate Europe, the proportion which they bear 

 to the Phanerogamous plants is as one to forty, while, in the 

 same circumstances, in the continental regions between the tro- 

 pics, Mr R. Brown admits that this proportion is as one to twen. 

 ty, and, in the least favourable cases, as one to twenty-six. 



