III. FLORA OF BRITAIN. . 399 



from ' Nyman's Sj'Uoge ' being also again accepted for the 

 European portion of it. By then adding Britain and 

 Yorkshire, two other floras are obtained for subordinate 

 portions of Europe and of our own island. These four 

 floras present the subjoined numbers : — 



Earth has 284 orders, 7854 genera, 82714 species. 

 Eui-ope 131 „ 1113 „ 9677 „ 



Britain 98 „ 496 „ 1425 „ 



Yorkshire 90 „ 404 „ 1000 „ 



Eeckoned on these numbers which are not rigidlj' re- 

 duced to one uniform standard (exceedingly laborious if 

 attempted, and barely within possibility, in respect to the 

 two first areas named) the average proportion of species 

 to an order and to a genus comes out thus : — 



Earth has 291.24 to an order, 10.53 to a genus. 



Europe 73.87 „ 8.60 



Britain 14.54 „ 2.87 



Yorkshire 11.12 „ 2.47 



It is thus made quite clear that with decreasing areas the 

 average number of species to a genus, and more especially 

 to an order, is largely decreased also. Yorkshire, with 

 nearly one-third of the orders, has only one-twentieth of 

 the genera, or thereabouts, and a very much smaller pro- 

 portion of the species. 



Now, if it be assumed that the groups of systematic 

 botany, orders and genera, do truly associate plants in 

 approximate accordance with their natural similarities, — 

 it is in this wise shown that the smaller the area, the 

 less similar are the plants which inhabit it ; that is to 

 say, they represent more orders and genera in proportion 

 to their absolute number of species ; the relative diver- 

 sity in the flora increasing as the space decreases. This 

 fact appears to bear upon certain views propounded in 

 regard to original centres. If there were originally 



