IX. ALTITUDE. 467 



Or, making the comparison between the numbers of the 

 absent species, instead of making it between the numbers 

 of those present in each stage of altitude : — 



Above 700 yards, .... absent 1163 species. 

 Between 700 and 200 yards, 738 



Below 200 yards, 106 



It is repeated, these numbei*s are apj)roximate only ; the 

 vertical ranges of various species being yet imperfectly 

 ascertained. Likely enough, as intimated on pages 230 

 and 231, several additional species will be found to occur 

 rather above 200 yards ; particularly those on the undu- 

 lating downs and hills in the south-easterly provinces of 

 England, which occasionally exceed 200 yards, and some 

 of them rising above 300 yards of vertical height. These 

 portions of the open country, distant from the higher 

 hills, have not been taken into account ; the altitudes for 

 the i)lants having been almost exclusively ascertained in 

 Middle and North Britain, with very few southern additions. 

 It is to be kept in recollection here, that difference of 

 latitude will tend to lessen the floral diversities at different 

 stages of elevation. Subject to some local exceptions 

 from other conditions interfering, it is familiaiiy known 

 to be a general rule or fact, that the lower limits of spe- 

 cies descend, and that the upper limits of species usually 

 become lowered, on mountains of equal bulk and eleva- 

 tion, as we trace plants in the polar direction to higher 

 latitudes and colder climates. The latitudinal extent of 

 Britain is amply sufficient to show this descent in vertical 

 limits, especially a descent of the lower limits of many 

 plants. Thus, at or very near the sea-level in North 

 Britain we may see Thalictnim alinniim, Carex capillans, 

 Dryas octopetala, Draba incana, Saxifraga oppositifolia, 

 and various other plants, which in Middle and South 

 Britain occur only above the lowest stage of altitude. 



