ls and also of plants occur in the
Alps. Of these it remains to be seen how many have come direct by way of
Northern Europe or from the Polar Regions by way of Asia. At any rate,
as the origin of the Alpine animals and plants will be discussed in
another chapter, there is no need to dwell on this subject at present.

From the nature of the distribution in Ireland of Arctic plants and
animals, which occur mostly on the north and west coasts, it would seem
that a stream of migration entered from Scotland, and I have no doubt
that that same migration came into Scotland directly from Scandinavia by
a route over which now roll the waves of the North Sea. There is,
moreover, as I already mentioned on p. 94, a very interesting so-called
American element in the north-western European flora, that is to say,
plants now found in North-west Europe and North America without
occurring in Greenland or any of the islands which might have formed the
former highway between the Old World and the New. These are probably
some of the more ancient Polar plants which have become extinct in the
Arctic Regions and survive in isolated patches in favourable localities.
We find seven species of these American plants in Ireland, almost
entirely confined to the north and west coasts. These are _Spiranthes
Romanzoviana_, _Sisyrinchium anceps_, _Naias flexilis_, _Eriocaulon
septangulare_, _Juncus tenuis_, and _Polygonum sagittifolium_. To them
must be added another plant recently discovered by the Rev. Mr. Marshall
in the south of Ireland, namely _Sisyrinchium californicum_. As I have
mentioned in former writings, there are three species of North American
freshwater-sponges in Ireland which have not hitherto been discovered
elsewhere in Europe or in Asia. These, namely _Ephydatia crateriformis_,
_Heteromeyenia Ryderi_, and _Tubella pennsylvanica_, all occur in some
of the lakes near the western coast of Ireland.

There are in all groups of animals instances of species which are
confined to Europe and North America, while unknown from the Asiatic
continent, but none, as far as is known, have such a very discontinuous
range as that of the animals and plants just referred to. In some cases
the species still occur in Greenland, and in this way make it still
clearer that their migration in former times took place from one
continent to the other by way of that country. As an interesting
instance of such distribution may be mentioned the Common Stickleback
(