454 X. GENERAL REMARKS. 



tude ; and we can therefore only compare the result of a 

 binary division in the one case, with the results of a ter- 

 nary division in the other case. Ireland was indeed 

 added, as a third and more westerly one, to the two lon- 

 gitudinal divisions of Britain proper, though without 

 admitting of any just comparison between their three 

 numerical totals ; the more westerly island being sepa- 

 rated by an intervening sea, having a much-restricted 

 latitudinal extension, and not having hills of equal eleva- 

 tion with those of Britain ; — conditions likely to affect 

 the flora much more than the difference of longitude 

 simply by itself. Though still open to some objections, 

 the ' census of species ' yields better data towards showing 

 that latitude operates greater floral changes than lon- 

 gitude. 



The efi'ect of the geographical differences before noted 

 between the three latitudinal divisions, pages 135, 136, is 

 an augmentation of the natural changes induced in the 

 flora by difference of latitude. In general terms, — not 

 subject to much exception, while considering an area so 

 small as Britain, — it may be said that difference of eleva- 

 tion affects the flora in a manner very similar to dif- 

 ference of latitude. Thus, the larger surface of low and 

 open country in South Britain, and the larger surface 

 and higher elevation of the mountainous ground in North 

 Britain, tend to increase the differences between their 

 respective floras ; and perhaps more particularly so in 

 respect to the non-identity of species. The addition of 

 arctic species to the mountain flora of North Britain, 

 such as occur on low plains only in latitudes farther 

 northward, must in part compensate numerically for an 

 earlier termination of some of the more southern species, 

 brought about by the repelling influence of the northern 

 mountains on such plants; — thus augmenting the posi- 



