igit. JREID. —BnVsh Plants and the Glacial Period. 207 



aquatic plants, why need we postulate a land-connection 

 for the land-plants, or a bridge of limestone to aid the 

 migration of the limestone plants from crag to distant crag ? 

 Aquatic plants and limestone plants must obviously in most 

 cases have taken leaps of many miles to arrive at their 

 present stations. Our plants have far greater power of 

 crossing deserts and seas than most botanists are willing to 

 allow. 



Let us examine the present distribution of one of the most 

 interesting groups of British plants. The Atlantic or Lusi- 

 tanian plants form an assemblage belonging mainly to the 

 Pyrenees, and found also in the S.W. of England, and again 

 in S.W. Ireland. But they do not occur in the intermediate 

 districts. If we look more closely into the composition of 

 this Atlantic flora, as it is represented in Britain, we lind 

 that only plants with small seeds have been able to cross to 

 Cornwall and Ireland, those with large seeds being left 

 behind on the Continent. There is only one tree among them, 

 and that is the Arbutus, one of the few trees with minute 

 seeds now living in Europe, h. further examination con- 

 fronts us with the puzzle that, whilst various Pyrenean 

 species are found also in Cornwall and Kerry, the species 

 occurring in Cornwall and Ireland are not the same. The 

 Arbutus is a case in point ; it is wild in Ireland, but in no 

 part of England. Erica ciliaris and E. vagans are English, 

 and not Irish ; E. mediterranea is Irish, and not English. 



The local distribution of these plants is equally strange. 

 A few, like Pinguicula lusitanica, have spread throughout 

 the west country, wherever the conditions are suitable. 

 Most occur, however, in quite different fashion ; they are 

 abundant over certain limited areas, to which they are 

 strictly confined, but they are absent from other adjoining 

 areas, though equally suited. I have mapped and examined 

 a good many of these areas, and the plants seem in most 

 places to be spreading vigorously from certain definite 

 centres, to which chance has transported a seed. Thus, 

 Erica ciliaris is confined to three localities in Cornwall, 

 Devon, and Dorset. E. vagans occurs abundantly in the 

 Lizard and again on quite different soil in North Cornwall, 



