INTRODUCTION. xliii 



greater antiquity tian Great Britain, or, to express tie same fact 

 in other words, to show that Great Britain maintained its land 

 connection with the Continent long after it had lost its land con- 

 nection with Ireland. As a consequence of this longer insulation of 

 the more western island, the stream of migration, hy which we may 

 assume the British Isles to have received from the Continent their 

 present plant poptdation, or, at least, the great bulk of it, at or 

 towards the close of the last Glacial Period, was, in the case of Ireland, 

 cut off much earlier than in the case of Great Britain. The advance 

 guard of aggressive species, the British type and a large section of 

 the English type, had time to push westward into Ireland before its 

 eastward land-connections were broken down ; but the rear-guard 

 of more slowly spreading species found their westward progress 

 checked by the land subsidence which created the Irish sea. The 

 mass of this rear-guard was probably formed of the Germanic type 

 plants, a group so little aggressive in character that it seems to have 

 been quite unable to push its way, as a whole, across England in 

 the face of the more hardy settlers who had gone before and 

 occupied the ground. 



Highland Type. — While we may find in such considerations as 

 these at least a partial explanation of the large deficiencies of the 

 Irish flora in the Germanic and English types, it is rather to 

 existing unfavourable climatic conditions that we should attribute 

 the chief influence in producing its deficiencies in the Highland 

 type. Not only are the Irish mountains inferior on the whole in 

 elevation to the "Welsh and Cumbrian mountains, but their distri- 

 bution is such as to prevent the production of the possible maxi- 

 mum of alpine conditions. The highest Irish summits, the 

 Magillicuddy's Eeeks, are placed in the extreme south-west of the 

 island, where latitude and the moderating influence of the large 

 adjacent ocean area combine to bring about perhaps the minimum 

 of alpine conditions which could be produced anywhere in Ireland 

 by a mountain mass of such elevation. In England, on the con- 

 trary, the Cumbrian summits, while little inferior in elevation to 

 the Eeeks, lie some 2 J degrees of latitude farther northward, and 

 the Snowdon group in Wales, with an elevation some 150 feet 

 greater than the Reeks, lies northward of them by a full degree of 

 latitude. In north Ireland, moreover, the mountain groups are 

 fully 600 feet inferior in elevation to the English groups under the 

 same parallel of latitude. Such being the position of the Irish 



