Vni. LATITUDE. 455 



tive floral diversity, by changing the species more, while 

 less altering their numbers. 



It appears by the ' census of orders,' that the numerical 

 values of the three floras run thus : — 



S. Britain, 1280. Mid Britain, 1148. N. Britain, 930. 

 Or, putting the comparison into another form, it may be 

 stated that, out of the total flora of Britain, the number 

 of species absent from those three latitudinal divisions 

 are these : — 



S.Britain, 145. Mid Britain, 277. N. Britain, 495. 

 It will be kept in recollection, that these are not compa- 

 rative estimates between the floras of different countries, 

 as described by different authors, and consequently varied 

 unequally by individual views about species and nativity. 

 They are here made between the several floras of tliree 

 divisions of a country well examined by many botanists, 

 and which are brought closely to an uniform standard in 

 respect to the distinctions between species, and those 

 between native and introduced plants. In this wise they 

 are good data towards illustrating the influence of lati- 

 tude, slightly augmented by that of elevation. 



The segregate species of JRuhus, it is true, are not so 

 well ascertained for North Britain, as for the other two 

 divisions ; and on the whole, the flora of that division is 

 less fully known than are the floras of the two more 

 southerly divisions. As a partial set off against this 

 numerical deficiency, it may be said that the flora of 

 North Britain is unduly increased by counting in it a 

 larger number of segregate species of Hieracium ; espe- 

 cially so, if compared with tbe flora of South Britain. 

 The boreal and montane genus Hieracium is thus made a 

 countei-poise against the austral and lowland genus Rubus. 



In the * census of species ' only binary divisions of lati- 

 tude are taken into account ; South Biitain against 



