ORIGIN OF THE BRITISH FLORA 213 



place by the bursting of the water through the low neck 

 of land between Calais and Dover, and the opening up 

 of a sea-passage to the west. When, after several alterna- 

 tions of cold and warmth, permanently temperate con- 

 ditions at last set in; the sea intervened between the 

 coasts of England and Prance, and it is hard to see how 

 the southern flora could have crossed this barrier, even 

 supposing that the width of the channel was much less 

 than it is to-day. It is a curious fact, however, that the 

 seeds of our Lusitanian plants are very small and light, 

 and include none of the larger kinds found on the Con- 

 tinent. It is possible, therefore, that some of these seeds 

 were driven across the narrow straits by the wind, 

 especially during storms, and, surviving the passage, 

 found a genial home on the other side of the Channel ; 

 others may have arrived on the feet of birds. 



The whole problem of the Lusitanian flora is very 

 difficult. At the root of the inquiry lies the debated 

 question of bygone climates and earth-movements, and 

 here the gaps in the record are so great that at present a 

 satisfactory solution is impossible. The difficulty is still 

 further comphcated by the presence in the west of Ireland 

 of several plants which are only found elsewhere in 

 America. 



The Lusitanian Flora, occurring in the west of Ireland 

 (or in Devon and Cornwall) and in the Iberian Peninsula, 

 comprises the following plants : 



London-pride (Saxifraga umbrosa), very common on the 

 rocks in the mountainous districts in the west of Ireland ; 

 the Pyrenean heaths {Erica Mackayi, E. mediterranea, and 

 Dabcecia poUfolia), all more or less abundant in the west 

 of Ireland ; the Cornish heaths (Erica vagans and E. 

 ciliaris) ■ the wild strawberry-tree (Arbutus Unedo), ex- 

 tensively cultivated in the south of England, but only 

 found wild round the shores of the beautiful Lakes of 

 Killarney ; Pinguicula gravdiflora, a butterwort found in 

 Cork and Kerry ; P. Ivsitanica, west of England and Ire- 

 land ; the Irish spurge (Euphorbia hiberna), found also in 

 Devon ; Saxifraga Oeum, west of Ireland ; S. hirsuta, Gap 

 of Dunloe, near Killarney ; Lobelia urens, Devon and 

 Cornwall ; Habenaria intacta, an orchid ; Helianthemum 

 guttatum, the spotted rock-rose, occurring, in Galway, 

 Cork, Anglesea, and Jersey ; Arenaria ciliata, on the 



