208 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



— at least if the biotype depauperization of the reHc community 

 has not been too extreme. 



The centre of origin of an area of other than a lower taxon, at 

 least in the absence of extensive palaeobotanical data, is apt to be so 

 difficult and hazardous of determination that little will be said about 

 it here. For its indication about a dozen criteria are commonly 

 employed, though sometimes a fair one may be given by isoflors, 

 which are lines delimiting regions supporting equal numbers of 

 species, e.g. belonging to a single genus. From the generic centre 

 outwards the number of species may be expected to decrease regularly, 

 and to assume a pattern which suggests the tracts of past migration, 

 which conversely may be followed backwards and found to converge 

 upon the generic centre. Even here, palaeobotanical confirmation 

 is wellnigh essential. Groups also have their single or multiple 

 centres of variation, where there are concentrated the greatest 

 diversity and wealth of forms (also called the inass centre), and their 

 centres of frequency, where there are accumulated the greatest 

 numbers of individuals or stations. 



With single species or lower taxa the situation may be far simpler. 

 Thus the tendency to decrease in the number of individuals of a 

 species towards the periphery of its area, is closely connected with 

 adaptability to definite habitat conditions. For whereas in the 

 centre of its area the habitat conditions most nearly approximate to 

 the optimum — so that the species can grow under fairly diverse 

 conditions, as it often does on different types of soil — nearer the 

 periphery of its area anything approaching this optimum is of 

 increasingly rare occurrence, and there is often lacking even the 

 minimum of conditions required for its normal existence. Thus 

 the European Beech [Fagiis syhatica), which ordinarily is capable 

 of growing on a variety of soils, is largely confined near the western 

 (moist) periphery of its range to the drier calcareous ones. 



As regards an area itself, its shape is best indicated on maps by 

 connecting all the peripheral points of its distribution. The shape 

 generally depends primarily on the physico-geographical conditions 

 of the country, and secondarily on the biological peculiarities of the 

 taxon involved. In the frigid and temperate zones the diameter of 

 most specific areas is much greater from west to east than it is from 

 north to south, whereas in the torrid zone species tend to have a 

 relatively larger latitudinal amplitude than elsewhere. 



An area of a species or subspecies usually comes into being largely 

 through migration and the operation of barriers thereto, the parent 



