AMERICAN STRANGERS 175 
there is a small but well-marked element in the flora 
which has its home in the northern portion of the 
New World; in our islands these species live side by 
side with the Pyrenean and Mediterranean plants 
lately dealt with. Here, then, is the problem set 
before us. How are we to account for the presence 
of these unexpected strangers in a flora derived in 
the main from a westward migration from the adjoin- 
ing parts of the Continent, from which they are 
absent? And especially what are their relations to 
the Glacial Epoch, during which the Continental flora 
was forced far southward by the advance of the ice, 
while that of our own islands was probably greatly 
reduced, and the balance forced into limited refuges 
in the south-west, if it survived at all? It should 
at once be pointed out that these peculiar Pyrenean 
and American elements in our flora are matched by 
similar elements in the fauna. Into the zoological 
evidence we cannot go here, but one well-marked 
species of each geographical group may be men- 
tioned. The Spotted Slug of Kerry (Geomalacus 
maculosus) is elsewhere confined to Portugal; while 
a little fresh-water Sponge, Heteromeyena ryder, 
widely spread in Irish lakes and rivers, and occurring 
also in Scotland, is otherwise exclusively American. 
In speculating, therefore, as to the origin of the 
plants, we must not leave out of account the question 
of the corresponding animals. 
First of all, is it possible that these unexpected 
organisms were introduced into our islands by man? 
In an earlier chapter it has been seen how human 
trade and intercourse have imported into our flora 
plants from the uttermost ends of the Earth. May 
