460 X. GENERAL REHARKS. 



Some other genera are common to North and Mid 

 Britain, though equally wanting in the South. Such are 

 Oxytropis, Slhhaldia, Ligusticum, Linncea, Arbutus, Trien- 

 talis, Corallorhisa, Tqfieldia, Elyna, and Sesleria. Few 

 years ago, Dryas might have heen included in this short 

 enumeration ; but by its recent discovery in Staffordshire, 

 and re-discovery in North Wales, it becomes a genus for 

 all the divisions. Actcea and Cypripedium are deemed 

 peculiar to Mid Britain ; possibly Asarum also. 



The absent genera in North Britain, that is, those 

 restricted to one or both of the two more southerly divi- 

 sions, are too numerous for citation. They can be ascer- 

 tained readily from the * summary of distribution,' if 

 required. And the absent species can be ascertained at 

 the same source for any of the divisions ; or, more closely 

 in accordance with their provincial and latitudinal limits, 

 from the ' areas of species.' Numerical summaries rela- 

 ting to the cessation of species under each degree of lati- 

 tude, and in each province from south to north, and 

 north to south, have been before given, on page 331. 



9. Relation of Plants to Altitude. 



Differences of altitude or elevation are marked by floral 

 changes somewhat less wide than those arising from dif- 

 ferences of latitude ; the alterations resulting from those 

 two geographical conditions being analogous, though not 

 altogether identical. In some respects the floral changes 

 might appear to be much wider in connexion with height 

 above the sea ; being brought about very rapidly in pro- 

 portion to the space of ground intervening between dif- 

 ferent levels. B}^ a railway journey from London to 

 Aberdeen, we may change our flora considerably in the 

 course of a single day ; but in order to effect that change, 



