ISLAND FLORAS. lxi 
ISLAND FLORAS. 
It is very noticeable that when an island is 
separated from the mainland by shallow channels, 
even though comparatively wide, the island flora 
corresponds very closely to that of the mainland. 
The flora of the British Isles differs very slightly 
from that of the continent opposite it. We may 
conclude that islands separated by shallow seas have 
been isolated for a much shorter time than those 
separated by deep channels. Great Britain has the 
“silver streak” we call the German Ocean, no part 
of which is as deep as Loch Lomond. We find, on 
the other hand, that the Azores, Madeira, and the 
Canary Islands, with a depth of many feet separating 
them from the coasts of Portugal and Morocco, have 
floras distinct from the nearest lands; and have 
flowers, the lineal descendants of late geological 
times, mixed with a few strays and waifs from the 
continent. Then Madagascar is separated from Africa 
by a broad and deep channel, and has a flora differ- 
ing much from the part of the continent of Africa 
opposite to it. : 
The flora of Orkney corresponds so closely to 
that of Caithness on the one hand, and of Shetland 
on the other, that as the floras of the three counties 
come to be better known, their differences are dis- 
appearing, eg., Ruppia rostellata, var. nana, when 
first discovered in the Oyce of Firth by Dr Boswell, 
was new to Great Britain ; now it has been found 
along the sea border of Sutherland and Ross. 
Elevation and nature of subsoil, depending on the 
