The Irish Naturalist October, 



vegetation was driven southward towards the Mediter- 

 ranean and its place taken by plants of the north. In 

 Ireland the case is not so simple, for the plants had not 

 an open line of retreat southward, since there lay the 

 Atlantic. The argument for the survival in Ireland of 

 at least part of the flora, based upon our interesting Lusi- 

 tanian, Mediterranean and North American plants, is 

 well known, and need not detain us here. There is a 

 tendency now among geologists to allow us what they 

 formerly denied us — ^a high land-level persist ng after the 

 ice had passed away. If this were of sufficient amount 

 and duration, it might have allowed of post-glacial immi- 

 gration of our southern plants (and the associated animals) ; 

 but it cannot be extended to allow of the overland migration 

 of the American element, for which a pre-glacial date still 

 appears essential. My friend Dr. Stapf, who follows Engler 

 in believing in the post-glacial arrival of our southern 

 forms, ^ agrees (in a recent conversation) that the American 

 element would appear to be of pre-glacial arrival. 



On the retreat of the ice, Switzerland became readily 

 re-colonized by much of its former vegetation, and no doubt 

 its mountain flora was permanently reinforced by many 

 arctic species which had been forced southward. Ireland 

 similarly became re-colonized, in this case from the east- 

 ward, so that the net result of the glacial upset is in both 

 countries a flora differing probably not much in type, 

 though somewhat in composition, from that prevailing 

 just before the oncoming of the Pleistocene cold. Ireland 

 s still occupied by a flora which is in the main a reduced 

 Swiss one. But the reduction is very great. Where 

 Ireland has 12 Trefoils, Great Britain has 18, Switzerland 

 24. Ireland has 3 species of Gentian, Great Britain 5, 

 Switzerland 20. Of Phyteuma Ireland has none, England 

 2, Switzerland 10. And so on, till the odds in favour of 

 Switzerland run into four figures, though its area is only 

 half that of Ireland. On the other side of the account, 

 we can point only to our Lusitanian -American group, and 



1 Otto Staff ; The Southern Element in the British Flora. Englers 

 Bot. Jahrbucher 50 (191 4) 509. 



