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 France, 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 igs 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 complicated 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 wmbrosa), very common on the 
rocks in the mountainous districts in the west of Ireland ; 
the Pyrenean heaths (Hrica Mackayi, EZ. mediterranea, and 
‘Dabecia polifolia), all more or less abundant in the west 
of Ireland; the Cornish heaths (Hrica 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 grandiflora, a butterwort found in 
Cork and Kerry ; P. lusitanica, west of England and Ire- 
land ; the Irish spurge (Hwphorbia hiberna), found also in 
Devon ; Saxifraga Geum, west of Ireland ; S. hirsuta, Gap 
of Dunloe, near Killarney ; Lobelia wrens, Devon and 
Cornwall; Habenaria intacta, an orchid ; Helianthemum 
guttatum, the spotted rock-rose, occurring in Galway, 
Cork, Anglesea, and Jersey; Arenaria ciliata, on the 
