in. FLORA. OF BRITAIN. 397 



S. Britain has 97 orders, 475 genera, 1820 species. 



M. Britain 94 „ 434 „ 1148 



N. Britain 79 „ 363 „ 930 

 The species for these and other sections of the island not 

 being yet ascertained with rigid exactness, the numbers 

 are of course to be received as approximative, not per- 

 fectly precise. Progressive knowledge will continually 

 render some changes needful in the figures which are 

 supposed to represent facts, but which in truth represent 

 only human knowledge about natural facts. Adopting 

 the above numbers as true for the present, the average 

 proportion of species to an order and to a genus will be 

 thus : — 



S. Britain has 13.19 to an order, 3.69 to a genus. 



M. Britain 12.21 „ 2.64 



N. Britain 11.77 „ 2.53 



It thus ajipears that the averages for all Britain give 

 more species to an order, and a fraction more to a genus, 

 than the averages for any portion of the island. Further, 

 a slight decrease is traceable from south to north. In 

 making comparisons between these decreasing proportions 

 or averages, it is to be recollected, first, that the areas of 

 the three divisions decrease northward, secondly, that the 

 numerical values of their three floras decrease north- 

 ward, and thirdly, that the temperature decreases north- 

 ward also. With which of these three differences the 

 decreasing averages have the closest relation, it may not 

 be easy to decide ; probably all three combiue to i)roduce 

 ■ even the small arithmetical results above indicated. 

 The comparison is caii'ied into larger figures, and per- 

 haps with less exactness, by making it between the flora 

 of Britain and the floras for considerable sections of 

 Europe, situate nearest to this island. Without looking 

 into the details of genera here, the numbers of orders 



