ORIGIN OF THE BRITISH FLORA 211 
Discontinuous Floras.—The present distribution of 
alpine plants is an illustration of what is known as a 
discontinuous flora. Such a flora is limited to certain 
widely separated spots, between which no plants of the 
same kind are found at all. Most plants have a wide 
range, being found, more or less frequently, in many 
adjacent countries ; but each plant has its limits of dis- 
tribution. These limits are determined by the barriers 
which it cannot pass—e.g., mountain chains, masses of 
water, unfavourable climate or soil. But although the 
presence of these barriers may explain the distribution 
of most plants, it does not explain the existence of plants 
in discontinuous areas. To understand this, we need to 
find out something of the past history of the land, its 
changes of contour, and alterations of level, as well as 
the secular changes of climate to which it has been sub- 
jected. All these things have produced in the past an 
ebb and flow of the vegetation, and with each change, 
physical or climatic, stragglers were left behind, which 
have given rise to the discontinuous floras of to-day. 
The Lusitanian Flora.—More striking still than alpines 
are certain plants found in the south-west of England. 
and in the west of Ireland, which do not occur elsewhere 
nearer than Spain and Portugal. For this reason the 
term ‘“‘ Lusitanian’”’ has been applied to these plants, 
from Lusitania, the old Latin name for Portugal. How 
are we to account for their presence in the British Isles ? 
It is not likely that they crossed the intervening sea, 
although it is possible that some may have been intro- 
duced by birds—e.g., Arbutus Unedo—by early invaders, 
or in merchandise by boats from Mediterranean ports. 
It is more probable, however, that most of these species 
arrived naturally by land during the time when the west 
of England was much nearer the coast of Europe than it 
is to-day. We know, as a fact, that Ireland was once 
not only joined to England and England to the Cov- 
tinent, but that to the south of our islands dry land 
extended at that time to the westward far beyond the 
present limits of France. Over this land, now sub- 
merged, low-lying, and enjoying a climate remarkably 
mild and moist, these plants may have crept up north- 
wards from Portugal to the south-western parts of 
Britain, and, conditions having remained suitable, they 
