GEOGRAPHICAL AND GEOLOGICAL BOTANY. 



vegetation cannot be explained by reference to existing influences, 

 such as transport, &c. Thus species with large seeds grow in 

 countries between which apparently insuperable obstructions to 

 transport exist ; many species are common solely to the tops of very 

 distant mountains ; many widely distributed aquatic plants pro- 

 duce seeds which sink to the bottom of the water when ripe, &c. 



Again, species are wanting in regions so well adapted to their 

 existence that, when arti ficially introduced, they establish them- 

 selves like natives. Certain countries, separated by broad oceans, 

 have more species in common than either the distance or nature of 

 their climate would render probable under ordinary circumstances; 

 while contiguous countries, with similar climates, sometimes pre- 

 sent very different species. 



Some countries are remarkable for a great number of species in 

 a given area, others for paucity of species. 



Lastly, species of simple structure (Rushes, Grasses, &c.) have 



often wide range, even though their seeds are not more easy of 



. transport than usual, while others of higher organization, with 



seeds or fruits easily transported (Composite, &c.) often occur in 



very limited areas. 



To sum up these statements, the occurrence of certain species, as 

 a general rule, in one region rather than another, their abundance 

 in particular localities, the extension and especially the disjunction 

 of species destitute of efficient means of transport, the non-exten- 

 sion, on the contrary, of species possessing seeds easy of transport, 

 certain analogies and certain differences between the floras of 

 several countries, and their relative richness in distinct forms all 

 these important phenomena are inexplicable by causes now in 

 active operation ; and we are consequently led to seek their solu- 

 tion by the aid of geological inquiries. 



The late Edward Forbes was the first to open this line of inquiry, in 

 a most acute and ingenious Essay on the Origin of the existing Flora of 

 Britain. Hooker has pursued the same line of reasoning in his inquiries 

 into the botanical geography of the southern hemisphere. 



Sect. 3. GEOLOGICAL INFLUENCES. 



Natural Science is incapable of elucidating the actual origin or 

 creation of organic beings ; but it seeks to trace up, as nearly as 

 possible, to the earliest periods the phenomena exhibited by 

 created things or beings, and the successive and gradual evolu- 

 tion of forms according to the laws of variation, the influence of 

 vital competition and external Conditions, as previously mentioned. 

 In addition, the botanist seeks, by geographical and geological in- 



