378 Relation of the Plant Population to the Glacial Period. 
occur, however, in quite different fashion ; they are abundant 
over certain limited areas, to which they are strictly confined. 
but they are absent from other adjoining areas, though equally 
suited. I have mapped and examined a good many of these 
areas, and the plants seem in most places to be spreading 
vigorously from certain definite centres, to which chance has 
transported a seed. Thus, Evica ciliaris is confined to three 
areas, in Cornwall, Devon and Dorset. &E. vagans occurs 
abundantly in the Lizard, and again on quite different soil in 
North Cornwall, so that the serpentine soil has nothing to do 
with its present distribution. 
Chance introductions of seeds during thousands of years 
explain the existing peculiarities of geographical distribution 
in a way that no changes of sea or land or climate will do. 
Our alpine flora consists largely of survivors from a colder 
period ; the rest of our flora, on the other hand, is constantly 
being added to by chance introductions from the nearest 
continental shore. That is why the Atlantic element, and the 
eastern element, though not consisting to any great extent of 
maritime plants, are confined mainly in Britain to areas within 
a few miles of the coast. Seeds are evidently brought from 
the Continent and scattered broadcast over certain coastal 
districts, and they grow and spread where soil and climate are 
suitable. But the post-Glacial period has been so short that 
the process is still incomplete, and the slow spreading inland 
has only as yet extended a few miles. We can still fix the point 
or points of introduction. 
The most striking elements in the British flora, except the 
arctic and alpine species, have a marked coastal distribution. 
The plants found correspond with those of the land opposite 
(in which they are often inland, as well as coastal). Thus 
the Cornish plants and those of S.W. Ireland contain a large 
Pyrenean element ; Norfolk plants correspond with those of 
the opposite shore of the North Sea; even two or three American 
plants are found on the coasts facing America. 
All the evidence seems, therefore, to point to a steady 
change and increase in our flora, due to occasional introductions. 
These introductions are, I think, now mainly due to birds 
driven by exceptional gales. But herds of migrating bison, 
deer, and horse have played their part, especially when the 
Straits of Dover were much narrower or non-existent. Packs 
of wolves which hunted the large game, foxes, cats, and especially 
raptorial birds which waited for and struck down the tired 
migrants, must also have assisted. Fences and the destruction 
of wild animals have probably rendered the process far slower 
than formerly ; but it still goes on, and anyone can see who 
notes the constant occurrence of seedling oaks miles from the 
nearest tree. 
Naturalist, 
