50. RETROSPECT. 



745 



elevation of new ones, is a demonstrated fact : and the 

 influence of such a change on the climates of particular 

 regions, if not of the whole globe, is a perfectly fair con- 

 clusion, from what we know of continental, insular, and 

 oceanic climates by actual observation. " Here, then," 

 observes Sir John Herschel, u we have, at least, a cause on 

 which a philosopher may consent to reason ; though whether 

 the changes actually going on are such as to warrant the 

 whole extent of the conclusion, or are even taking place 

 in the right direction, may be considered as undecided, till 

 the matter has been more thoroughly examined."* 



50. Retrospect ; Botanical epochs. — I will conclude 

 this discourse with a review of the prevailing botanical 

 characters during the principal geological epochs. 



Count Sternberg, M. Adolphe Brongniart, Dr. Lindley, 

 and other eminent botanists, have adduced some interesting 

 generalizations from the fossil floras of the various forma- 

 tions : and although conclusions of this kind must be 

 regarded in the nature of shifting hypotheses, and will 

 require to be modified by new discoveries — for all the fossil 

 species at present known amount to but two thousand*]* — 



* Discourse on the Study of Natural Philosophy ; p. 146. 

 f M. Goppert computes the number of fossil plants known, to 

 amount to nearly 2,000. Their mineral distribution in the strata is 

 stated to be as follows : — 1 



In the palseozoic strata, below the coal, principally 

 in the Devonian, for the Silurian only contain 



a few fucoids 52 



Carboniferous 819 



Permian 58 



Triassic 86 



Oolitic 234 



Wealden 30 



Cretaceous 62 



Tertiary 454 



1 Brit. Assoc. Reports, for 1845. 



