622 HISTORY OF GEOLOGY. 



are found to be determined in their situation by 

 more general conditions. Thus it is the character 

 of the flora of a collection of islands, scattered 

 through a wide ocean in a tropical and humid 

 climate, to contain an immense preponderance of 

 tree-ferns. In the same way, the situation and 

 depth at which certain genera of shells are found, 

 have been tabulated * by Mr. Broderip. Such gene- 

 ral inferences, if they can be securely made, are of 

 extreme interest in their bearing on geological spe- 

 culations. 



The means by which plants and animals are 

 now diffused from one place to another, have been 

 well described by Mr. Lyell 3 . And he has considered 

 also, with due attention, the manner in which they 

 become imbedded in mineral deposits of various 

 kinds 4 . He has thus followed the history of organ- 

 ized bodies, from the germ to the tomb, and thence 

 to the cabinet of the geologist. 



But, besides the fortunes of individual plants 

 and animals, there is another class of questions, of 

 great interest, but of great difficulty ; the fortunes 

 of each species. In what manner do species which 

 were not, begin to be? as geology teaches us that 

 they many times have done ; and, as even our own 

 reasonings convince us they must have done, at least 

 in the case of the species among which we live. 



We here obviously place before us, as a subject 



2 Greenough, Add. 1835, p 20. 



3 Lyell, B. iii. c. v. vi vii. 4 B. iii. c. xiii. xiv. xv. xvi. 



