Relation of the Plant Population to the Glacial Period. 377 
clay or by sand is as much an island, as far as many of our 
most peculiar plants are concerned, as if it were surrounded 
by water. We have many such islands—or oases is perhaps 
a more suitable term for them—and no possible ups and downs 
of the land will connect them. Many of them, like the central 
limestone district of Ireland, or the Peak District in Derbyshire, 
or the West Yorkshire Carboniferous limestone, must have been 
isolated from far-distant geological periods, from times before 
the present flora of Britain had any existence. We have a still 
more difficult problem than this. Britain is divided into 
numerous river-basins, for most of which any connection 
with other basins in post-Glacial times is unthinkable. Yet 
each basin yields numerous aquatic plants and animals of the 
same species as those found in other basins cut off by high 
hills. Isolated lakes have their aquatic flora; and even 
artificial ponds, such as the dew-ponds of our high chalk downs, 
have a fauna and flora closely proportionate in the number of 
species with the time that has elapsed since the pond was 
made, or since it last dried up. If no actual connection between 
river-basins or isolated ponds is needed for the spread of 
aquatic plants, why need we postulate a Jand-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 find 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. A further examination confronts 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. Evica ciliaris and E. 
vagans are English, and not Irish ; EF. 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 
1911 Nov. 1. 
