7] TYPES AND AREAS OF NATURAL DISTRIBUTIONS 201 



have become drastically reduced through the activities of Man, and 

 the ' cultivated relics ' whose sown area, owing to their low economic 

 value, has been reduced to small compass and few localities, we 

 may usefully distinguish three main classes of relics on the basis 

 of the type of natural habitat-change which has isolated them. 



( 1 ) Formation relics that occupy limited areas within the boundaries 

 of major plant communities (formations) which have undergone 

 considerable changes in composition. Striking examples are the 

 residual wooded tracts that are sometimes found in some extensive 

 grasslands. 



(2) Geomorphological relics that are connected in their habitat 

 preferences with particular ecological conditions but that, owing to 

 edaphic or allied changes, are no longer provided with the conditions 

 of growth to which they are accustomed. Familiar examples include 

 marine plants inhabiting freshwater lakes, and shore plants growing 

 along the edges of clried-up gulfs. 



(3) Climatic relics that give evidence of having originated and 

 formerly flourished under other climatic conditions than those in 

 which they now grow. Examples are the mesothermic plants to 

 be found in some boreal areas that have cooled at least since the 

 ' postglacial optimum ' when such plants presumably migrated to 

 these areas. 



While it is scarcely possible to distinguish further classes of 

 ' biotic relics ' resultant on grazing, etc., as distinct from those 

 engendered by Man or his animals, or on plant competition with 

 our present limited knowledge thereof, there is another basis on 

 which relics may be classified, namely, their age and origin, the 

 main such classes being : (i) pre-Tertiary, (2) Tertiary, (3) glacial, 

 (4) interglacial, and (5) postglacial relics. 



Vicarious Areas 



These are areas belonging to closely-related taxa (vicariads) 

 derived from the same common ancestor and tending to be mutually 

 exclusive of one another in naturally {i.e. without human interference) 

 occupying separate areas. Sometimes, and especially when their 

 ecological requirements differ only very slightly, vicariads may be 

 mutually exclusive through being closely competitive. This is the 

 case with many subspecies and closely-related species ; on the other 

 hand, with higher groupings — and even families and whole com- 

 munities may in a sense be vicarious — there is less reason to suppose 



