Academy of Sciences in Paris. 343 



that the vegetable remains found in the most ancient deposits of 

 the globe are plants belonging specially to the families of the 

 equisita, ferns, and lycopodia; that it is not until a higher series 

 of formations about the mottled freestone (gres bigarre) that some 

 coniferce are found ; the cycades are only found still higher, and 

 the dicotyledonous plants do not appear until immediately after 

 the chalk. The primary vegetation of the globe, therefore, consisted 

 of vascular cryptogamiae, with about one fifteenth part of their number 

 of monocotyledonous phanerogamic, and these plants were of immense 

 size ; the horse-tails or equisita grew to ten feet in height and from 

 five to six inches in diameter ; the ferns from 40 to 50 feet in height, 

 and the lycopodia from 60 to 70 feet. When the coniferoe begin 

 to appear, the cryptogamias become less numerous ; the species are 

 no longer the same, nor have the same magnitude. When the 

 remains of cycades commence, the species of the cryptogamiye are 

 again different, several genera have entirely disappeared, and the 

 number of these plants, which in the first epoch bore a proportion to 

 that of the monocotyledonous phanerogamiae of 14 to 1, is now only 

 about equal to it : so that in the primitive floral world the cryptoga- 

 miae formed -||- of the whole number of plants ; in the middle epoch 

 they constituted only half, and the other half was composed of coni- 

 feroe and cycades ; while in the present distribution of plants on the 

 surface of the earth these families scarcely form one three-hundredth 

 part of the whole number. When the dicotyledonous plants appear in 

 the beds of the earth, their number becomes suddenly immense, and 

 the cryptogamiae, which then belong to genera different from those 

 found in the former beds, disappear almost entirely. The numerical 

 relation of the different families with each other then becomes nearly 

 similar to that of the plants now existing on the surface, and the 

 most numerous species are those which have the strongest ana- 

 logies with living plants. Hence it appears that the vegetation 

 of the earth has greatly changed at different epochs, and has 

 become more and more complex, so that the long lapse of time, 

 during which all the deposits have been formed, is divided into various 

 periods of unequal length, during each of which vegetation had 

 peculiar characteristics uniform throughout the earth, and after each 

 of which vegetation completely changed in its genera, its families, 

 and even its classes, as well as in the numerical relation between 

 the different species, until it ultimately arrived at a point nearly 

 resembling that which now exists. It is also remarkable that the 

 beds in which are found the remains of plants of any given period 

 are separated from those containing the remains of plants of a dif- 

 ferent period by deposits entirely destitute of terrestrial plants ; whence 

 we may conclude that there were periods of repose, during which 

 the surface of the earth was wholly unproductive, a circumstance 

 which has an immediate relation with the differences observed 

 between the various periods of vegetation, which indicates the sudden 

 convulsions by which every thing then existing was destroyed, ami 

 VOL. II. Nov. 1831. 2 A 



