II. FLORAL AREAS. 377 



Macculloch. So that, if every known species of flowering 

 plant and fern could be brought into Britain, this one 

 island would afford upwards of a square mile of surface 

 for each species of plant. But Britain is only a small 

 portion of the whole earth ; and its own flora includes 

 little more than a sixtieth part of the whole number of 

 species alleged to be known. Supposing the known flora 

 of the earth to be still far short of completeness, and that 

 it may eventually be doubled in its numbers, through new 

 discoveries and greater subdivision into segregate species, 

 an average area of many square miles on the earth would 

 still remain for each species. Within the narrower limits 

 of Britain the 88,000 square miles of surface, set against 

 1425 species, would also allow an area of many miles to 

 each species singly; that is, each species might have 

 more than sixty miles of the surface to itself, if evenly 

 divided. But no botanist requires to be told that an area 

 of sixty square miles in England will produce some hun- 

 dreds of species. Indeed, an area of one single square 

 mile will be found to contain (say) 50 to 300 species, and 

 occasionally even 400, according to fertility of soil and 

 variety of local stations. 



It is thus made veiy obvious that the smaller the area, 

 the more numerous is the flora relatively to the space. 

 This arises in the fact of many species being spread over 

 wide areas, and being consequently counted many times 

 over, in reckoning up the various floras for included 

 spaces of much more limited extent than their own areas. 

 In dividing Britain into two longitudinal divisions, 

 western and eastern, comparatively few species were not 

 counted in the flora of both. In making three latitudinal 

 divisions, a majority of the species are found to be com- 

 ponents of their three floras alike. In the eighteen pro- 

 vincial districts, full three hundred species (003, by the 



VOL. IV. . 3 c 



