BOOK IV 



BOTANICAL ECOLOGY OR TOPOGRAPHY 



WHILST the climatic conditions, upon which the floristic 

 regions chiefly depend, are in the main uniform over 

 the whole of those regions, the edaphic conditions com- 

 monly vary within much smaller areas. In the more 

 minute or topographical study of plant-distribution it is, 

 therefore, largely upon these edaphic conditions that 

 we depend for the definition of smaller areas. In other 

 words, among the factors which produce the vegetation 

 of these smaller areas edaphic influences are the most 

 effective or master factors. At the same time it must 

 never be forgotten that climatic and edaphic factors com- 

 bine to produce the vegetation. The greater prevalence 

 of Ferns, for instance, in the west of Britain is due to the 

 humidity of the air and not to any peculiarity of soil. 



Ecology is the study of habitats, or environment, and 

 the response thereto, as shown by the nature and 

 characteristics of the inhabitants. The habitat is 

 obviously the sum of all the external influences brought 

 to bear on the organism. 



Whilst in the broader study of plant-geography we 

 constantly deal merely with the occurrence of species in 

 various regions, so that a scarce plant is treated often 

 on an equal footing with the commonest form, in this 

 topographical study we are concerned rather with in- 

 dividuals, so that prevalent or " dominant " species 

 which give a " physiognomy " or " facies " to the land- 

 scape are dealt with and the rarest occurrences are 

 overlooked. This is sometimes expressed by terming 

 ecology the study of vegetation ; but, obviously, in this 

 study it is impossible to separate the study of the environ- 

 ment from that of the vegetation that responds to it. 

 The precise modes of action of such external conditions 

 as salt, or lime, or humus, are questions for the physiolo- 

 gist. The external modifications of form resulting from 

 various conditions, which we have already to some 

 extent discussed, are of importance to the geographer 

 because they influence the character of the landscape. 



