The principal factors which give rise to slow or sudden 

 changes in the natural vegetation are: 



successive cold seasons which invite northern and drive 



out southern species; — 

 successive warm seasons which do the reverse; — 

 very wet or very dry seasons which cause the filling 



or emptying of ponds; — 

 the natural sanding in of lakes, narrowing their borders ;— 

 extension and regression of the sand-dunes; — 

 influence of salty and sulphury water from springs; — 

 changes in the scattering of seeds by wind and birds 



and altered water-courses; — 

 the cutting of roads and canals; — 

 the cutting of pine-woods for turpentine or lumber, 

 which is going on over large areas, and which modifies 

 the amount of moisture and shade; — 

 the taking of land into cultivation, for truck or citrus- 

 planting; — 

 and other less visible causes for the appearance of some 

 species and the disappearance of other species, for 

 instance along the roadside in and near the towns. 



