Ancient Vegetation of the Earth. 323 



the earth was subjected during this period ? The operations of 

 nature now going on in different regions of the globe, may throw 

 some Ught upon this question. 



The study of the geographical distribution of those plants ap- 

 pertaining to the same families which alone composed the vege- 

 tation of the coal period, may, indeed, indicate to us the climac- 

 terick conditions and consequently the physical causes which 

 favoured the increase of stature as well as the great frequency of 

 these vegetables ; and we may conclude from these, with much 

 probability, that the same causes determined their preponderance 

 at that epoch. 



We see, for example, that the Ferns, Equiseta, and Lycopodise 

 attain a more lofty stature in proportion as their geographical po- 

 sition approaches the equator. Thus it is only in the hottest 

 regions of the globe that we find those arborescent Ferns which 

 combine with the towering and majestick mien of the Palms, the 

 elegant foliage of the ordinary Ferns, and of which we have 

 indicated the existence in the coal formations. In these same 

 regions the Equiseta and Lycopodias attain to a stature double or 

 triple that which the largest of these species present in temperate 

 climates. A second condition appears to have a still more marked 

 influence upon their preponderance, in reference to the vegetables 

 of other families, namely, humidity and uniformity of climate ; 

 conditions which are united in the highest perfection, in the small 

 islands situated far distant from continents. 



In such islands, indeed, the extent of the surrounding oceans 

 fixes a temperature with but slight variations, and coupled with 

 perpetual humidity ; circumstances which appear to favour, in a 

 remarkable manner, the development and the variety of specifick 

 forms among the Ferns and the analogous plants; while, on the 

 contrary, under the inflaence of the same conditions, the pheno- 

 gamoiis plants are little varied, and are far less nimierous. From 

 these causes it results, that while on the extensive continents of 

 the earth the vascular Cryptogamick plants, such as the Ferns, 

 Lycopodiee, Equiseta, &c. often form scarcely one fiftieth of the 

 total number of vegetables, yet in the small islands of the equa- 

 torial regions, these same plants constitute almost half, and in 

 some cases, even two thirds of all the vegetables which inhabit 

 them. 



