Habitat and the Classification of Vegetation 13 



formations are found only where intermediate habitats 

 occur, and are, on the whole, rare and restricted in area, 

 because the great habitats depending on soil are essentially- 

 distinct units, except in those cases in which a funda- 

 mental and progressive change in the habitat is going 

 on, e.g. in the gradual conversion of an aquatic into a 

 terrestrial habitat. In such cases we have to recognise 

 the gradual supplanting of one habitat by another. Man 

 has, in general, no power to change the formation, except 

 by changing the whole character of the habitat, e.g. by 

 draining marsh land, or by substituting planted crops for 

 the natural vegetation. 



The association, on the other hand, is determined by 

 minor differences of habitat or by the course of succes- 

 sion, and is often completely changed when human 

 interference takes place. Transitions between associations 

 are very frequent, owing to the gradual nature of pro- 

 gressive and retrogressive changes, and to the variation 

 from place to place of the minor features of the habitat. 



Finally, plant-societies are minor features of vegetation, 

 and their presence in certain spots is generally determined 

 by some biological peculiarity of a species, not by the 

 habitat as such. 



The classification of the units of vegetation here 

 adopted cuts across the superficial classification based 

 on plant-form, physiognomy, or mere topography. Thus 

 we do not divide vegetation into groups of tree-formations, 

 shrub-formations and herb-formations, because species 

 belonging to all these plant-forms, and associations 

 dominated by these species, actually occur ia one and the 

 same general type of habitat. 



Habitat must be the basis of any natural classification 

 of vegetation because it is on similarities and differences 

 of habitat that vegetation ultimately depends. It is true 

 that plant-forms have an important relation to habitat. 



