212 INTRODUCTION TO PLANT GEOGRAPHY 



to a humid maritime climate and those which prefer an arid con- 

 tinental one of marked temperature extremes. The oceanic elements 

 are generally considered to be the more ancient, the initial land 

 flora having been evolved from an aquatic one and further evolution 

 having been along lines of emancipation from dependence on water 

 and, accordingly, adaptation to more continental habitats. Such 

 ecological elements can be of great significance in elucidating the 

 history of a particular flora and any major vagaries of climate to 

 which it may have been subjected. It should be remembered, 

 however, that within the limits of a country or natural region there 

 may be found tracts, such as mountain massifs, in which particular 

 conditions predominate and which accordingly give refuge to ' alien ' 

 plants. These may be termed inclusions, in contradistinction to the 

 basic element of types properly belonging to the floral region, and 

 the more general penetrants from outside. 



Major Regions 



These will be considered here only in the broadest outline, as 

 several subsequent chapters are devoted to them. Vegetational 

 regions, being based on life-form rather than on taxonomic proximity, 

 may cut across the ranges of systematic units and also differ greatly 

 from zoogeographical realms characterized by particular animal 

 communities. Fig. 65 indicates the main vegetational-climatic 

 regions of the world in highly generalized form, and is the basis 

 of the division followed in Chapters XII et seq. It may be noted 

 that the western Old World desert region, which is sometimes 

 separated as a special one, is here included in the tropical region, as 

 the Mediterranean is in the temperate region. This gives us a 

 central tropical belt and, to both the north and the south, two others. 

 These are the temperate (in the wide sense) and polar belts, the 

 former ranging approximately from the polar tree-line and including 

 the subarctic and warm-temperate zones, while the tropical belt con- 

 veniently includes the subtropics. Each of these broadest of regions 

 is itself complex, tending to show latitudinal gradation — so much so 

 that accurate detailed maps are scarcely conceivable, at least in our 

 present state of frequent ignorance of local features. 



Further Consideration 



Most of the subjects dealt with in this chapter are further discussed 

 by Wulff and Cain in their works cited at the end of the preceding chapter. 



