If we suppose that an extensive centre of origin existed in the Polar area, or we may say in Greenland, both animals and plants would have been able to spread from it into Northern Europe and North America by means of the land-connections which are generally supposed to have existed in pliocene times, that is to say, just before the commencement of the Glacial period. There must have been at this time a connection too between Scotland and Scandinavia, which will be dealt with more fully presently. The important point is to consider what light the Greenland flora and fauna will throw upon the problem of the continuity of the aforesaid land-connection during the Glacial period. We have seen that the Barren-ground Reindeer, a typically Polar species, penetrated as far south as the Pyrenees, the Arctic Hare went as far, while a number of other species of Polar animals 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.