74 THE SCOTTISH BOTANICAL REVIEW 



of the surface and superficial leaching are of less importance 

 provided the deeper soil-waters retain their alkalinity. 



As shown by Graebner (2), heathland has developed within 

 the present sphere of the climatic influence of the woodland 

 province through leaching of the surface layers of the soil 

 where these are more than usually porous.^ Within this 

 province moorland has a physiographic or topographic 

 distribution, while within the moorland region heaths closely 

 resembling those of the heathland proper may arise through 

 physiographic causes preventing humus accumulation, or 

 forwarding its aeration and disintegration as great porosity 

 of the surface, steepness of slope, and exposure to insolation 

 or wind. With an expansion of the moorland province a 

 greater area of the heathland would doubtless progress to 

 moorland, and the former might therefore be looked upon as 

 a delayed or abortive stage of moorland. The heathland in 

 the south-east of this country is evidently an extension of 

 the heaths of the N. German Plain, as pointed out in the 

 " Types of British Vegetation." Other heathland areas in 

 the north and west bound and intersect the moorland, and 

 are evidently abortive or retrogressive phases of the latter. 



The heathlands of the N. German Plain and S.E. Eng- 

 land cover a district which appears to have been peculi- 

 arly subject to marked changes of vegetation in post-glacial 

 times. During stages of Continental extension and summer 

 drought, the moorland province contracted towards the 

 Atlantic border, and wide areas of peat-covered ground were 

 invaded by pine-forest. At the same time the Russian 

 steppes probably advanced far into the N, German 



^ I have used the terms " moorland province" and "woodland province "to 

 denote the areas in which the climate tends to the dominance or decadence of 

 moorland or woodland as the case may be; the terms "moorland region" and 

 " woodland region " to comprise the areas where either moorland or woodland 

 is the dominant natural vegetation at the present time ; and the term "district" 

 for separate areas occupied by stable plant-formations within such pro- 

 vinces or regions. The province shifts with secular changes of climate, but, 

 owing to physiographic or edaphic factors, plant-formations, which stabilised 

 within a certain climatic province, may persist in a retrogressive condition, for 

 shorter or longer periods, beyond its shifted boundaries ; and owing to similar 

 factors plant-formations differing widely from the dominant vegetation of a 

 climatic province may arise and hold their own within such a province. Stable 

 plant-formations might thus be classified as (i) dominant or climatic, where the 

 prevailing factors are existing climatic factors ; (2) retrogressive or relic, where 

 the prevailing factors are past climatic factors ; (3) topographic, where the pre- 

 vailing factors are physiographic and edaphic ( 19). 



