INTEODTJCTION. xlv 



at the present day of a littoral flora in Greenland, wliere tte oon- 

 ditions may te taken as fairly typical of those wliicli obtaiaed ia 

 Ireland during the Glacial Period, renders it not, improbaWe that 

 a similar flora made up of Highland, that is Arctic or sub-Arctic 

 species, may have maintained itself in some favoured spots along 

 our sea-board, while the country, as a whole, lay buried under an 

 ice-sheet. If we conceive the Irish Highland T3rpe plants of to- 

 day to be derived from such a remnant of our ancient flora, main- 

 taining itseli chiefly along the western Atlantic sea-board, where 

 the climatic conditions were probably most favourable during the 

 last Glacial Period, and retreating to its present mountain stations 

 on the advent of a milder climate, then the absence from East 

 Ireland of the 12 Highland species common to West Ireland and 

 West England may become somewhat less perplexing. But in 

 considering this, as all other problems in plant distribution it must 

 always be borne in mind that the causes ia operation have been 

 highly complex. Past geological changes; present climate and 

 configuration of the surface ; accidental or occasional means of 

 transport across sea-channels ; constitution of soils : all of these 

 have had a share in bringing about the existing disposition of 

 the Irish Highland flora. 



The following ten species of this Highland group, or just one- 

 fourth of the total number, descend in Ireland to, or almost to, 

 eea-level : — 



Draba incana. Galiuic boreale. Juniperua nana. 



Dryas octopetala. Hieracium anglicum. Sesleria oaerulea. 



Saxifraga aizoides. H. iricum. Selaginella selaginoides. 

 Sedum Bhodiola. 



Scottish and Intermediate Types. — ^As was shown ia the com- 

 parative table on p. xlii, 64 per cent, of these types, combined, 

 representing the boreal element in the English flora, is found in 

 Ireland. The Irish plants of these types are set out in the 

 following list : — 



Scottish and Intermediate Type Plants in Ireland. 



Thalictrum minus. Arenaria verna. Potentilla frutioosa. 



Trollius euiopsBus. Geranium sylvatiomu. Rubus saxatilifl. 



Helianthemum canum. Vicia Oiobus. Poterium officinale. 



Viola lutea. V. sylvatioa. Rosa hibernioa. 



Sagina eubulata. Pmnus Padus. Saxifraga Hiroulus. 



