II] GEOGRAPHICAL DISTRIBUTION 31 



mediterrmiea) and Erica Maclaii Avhicli are not 

 found elsewhere in the British Isles or in the whole 

 of northern Europe, but reappear in the P3a'enees. 

 The London Pride (Saxlfraga umhrosa), another 

 Pyrenean plant, grows on the south and west coast 

 of Ireland from Waterford to Donegal. Arbutus 

 Unedo, the Strawberr}^ tree, which flourishes in the 

 Killarney district of County Kerry and occurs in 

 neighbouring localities, has a Avide distribution in 

 the Mediterranean region. Devonshire and Cornwall 

 possess two other Heaths, Erica ciliaris, which ex- 

 tends into Dorsetshire and occurs in north Brittany, 

 and Erica vagans, both Pyrenean species, while a 

 ]\Iediterranean plant. Gladiolus illyriciis, grows in the 

 New Forest. 



In 1846 Edward Forbes dealt with the problems 

 presented by the distribution of British plants in an 

 essay which has exercised a far-reaching influence. 

 When Forbes published his work, comparatively little 

 was known as to the possibilities of transport of seeds 

 and fruits across barriers of water (22). His conviction 

 that the known means of dispersal were insufficient 

 to account for the presence of Mediterranean or 

 Lusitanian plants in Ireland led him to turn to 

 geology for a solution of the problem. He was thus 

 led to put forward the view that in the course of 

 the Tertiary period when, as we know from palae- 

 ontological evidence, the climate of north and west 



