EPOCHS OF THE GLOBE. 481 



dicotyledons a steady progression from those which we may term of a 

 lower to those of a more elevated organization, and all this was produced 

 Iby the influence of physical agents. 



On so firm a footing may we regard this doctrine as now placed, that 

 we can use it for the purpose of determining from the ascer- . ,. .. 

 tained botanical condition of our planet at any period the these principles 

 physical conditions under which she then existed, and this inverse1 ^- 

 with a precision constantly becoming greater. Among the more impor- 

 tant facts which have been distinctly made out, a few may be cited as 

 illustrations of the subject now treated of. For example, 1st. The ex- 

 istence of a tropical climate in regions of very high latitude, as is proved 

 by the occurrence of fossil tropical plants therein ; 2d. That all over the 

 globe the temperature was once nearly uniform, nothing answering to 

 what we now term climates existing, as is proved by the uniformity of 

 the vegetable growths preserved as coal from the equator to near the polar 

 circles great arborescent cryptogamia, exceeding in size the arborescent 

 ferns now growing in the Pacific islands under the equinoctial line. 

 From such a botanical fact, we reason without error to the Succession of 

 conclusion that in those times the influence of the sun, so climates on the 

 far as the supply of heat was concerned, must have been ined from its 

 wholly overpowered, the intrinsic temperature of the planet fossil flora - 

 obliterating all climate subdivisions. 3d. That these climate subdivis- 

 ions, which are now presented as existing side by side in zones upon the 

 planet, were introduced for each latitude in an order of succession as to 

 time ; that even the frigid zone, by reason of the cooling of the earth, 

 has passed through an ultra-tropical, a tropical, and a temperate degree 

 of heat to reach its present state ; 4th. That the extinction of the old 

 vegetable forms was accomplished by an inability of those organisms to 

 maintain themselves in the physical revolution that was gradually taking 

 place. Among such may be mentioned the dying out of gigantic equi- 

 setums or horsetails twenty feet high, club mosses rivaling forest trees, 

 calamites and stigmarias, these, as they disappeared, being replaced by 

 cycadacese, and coniferge, and tree-like liliacese. Even long after the de- 

 posit of the coal there flourished in England innumerable palms, which 

 maintained themselves, with their tropical associates, into the tertiary 

 times. 



Among the physical events which geological researches disclose, there 

 are two of surpassing importance in the history of the globe, Two epochs in 

 and both of them immediately connected with the doctrine the history of 

 we have under discussion ; these are the change impressed * e g 

 on the atmosphere by the withdrawal from it of those enormous masses 

 of carbon deposited under the different forms of coal, and the localiza- 

 tion of plants and animals in climate distribution as the sun's rays be- 



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