In his comments on this extensive list of supposed peculiar British plants, Sir Joseph Hooker arrives at the following conclusions:—
1. There are four unquestionably distinct species which do not occur in continental Europe: viz.—
One absolutely endemic species, Potamogeton lanceolatus.
Three American species, Sisyrinchium angustifolium, Spiranthes romanzoviana, Eriocaulon septangulare.
2. There are sixteen endemic varieties of British species, viz.—
Eleven of more or less variable species, Caltha palustris, var. radicans; Polygala vulgaris, var. grandiflora; Cerastium arcticum, var. edmonstonii; Trifolium repens, var. Townsendii; Rosa involuta, var. wilsoni; Rubus fruticosus, sub-sp. longithyrsiger; Campanula rotundifolia, var. speciosa; Erythræa centaurium, sub-sp. latifolia; Carex involuta, (? Hyb.); Carex vesicaria, var. Grahami; Deyeuxia neglecta, var. Hookeri.
Five of comparatively well limited species. Arabis petræa, var. grandifolia; Helianthemum guttatum, var. Breweri; Geranium sanguineum, var. Lancastriense; Œnanthe Phellandrium, var. fluviatilis; Spartium stricta, var. Townsendi.
The above twenty species are marked in the list with an asterisk. Of the remaining fifty-five, Sir Joseph Hooker says, "that for various reasons it would not be safe to rely on them as evidence. In most cases the varietal form is so very trifling a departure from the type that this may be safely set down to a local cause, and is probably not constant. In others the plant is doubtfully endemic; in still others a hybrid."
Even should it ultimately prove that of the whole number of the fifty-five doubtful forms none are established as peculiar British varieties, the number admitted after so
rigorous an examination is about what we should expect in comparison with the limited amount of speciality we have seen to exist in other groups. The three American species which inhabit the extreme west and north-west of the British Isles, but are not found on the continent of Europe are especially interesting, because they demonstrate the existence of some peculiar conditions such as would help to explain the presence of the other peculiar species. Whether we suppose these American forms to have migrated from America to Europe before the glacial epoch, or to be the remnants of a vegetation once spread over the north temperate zone, we can only explain their presence with us and not further east by something favourable either in our insular climate or in the limited competition due to our comparative poverty in species.
About half of the peculiar forms are found in the extreme west or north of Britain or in Ireland, where peculiar insular conditions are at a maximum; and the influence of these conditions is further shown by the number of species of West or South European plants which occur in the same districts.
We may here notice the interesting fact that Ireland possesses no less than twenty species or sub-species of flowering plants not found in Britain, and some of these may be altogether peculiar. As a whole they show the effect of the pre-eminently mild and insular climate of Ireland in extending the range of some south European species. The following list of these plants, for which I am indebted to Mr. A. G. More, with a few remarks on their distribution, will be found interesting:—
List of Irish Flowering Plants which are not found in Britain.
1. Polygala vulgaris (var. grandiflora). Sligo.
2. Campanula rotundifolia (var. speciosa). W. Ireland.
3. Arenaria ciliata. W. Ireland (also Auvergne, Pyrenees, Crete).
4. Saxifraga umbrosa. W. Ireland (also Pyrenees, N. Spain, Portugal).
5. ,, geum. S. W. Ireland (also Pyrenees).
6. ,, hirsuta. S. W. Ireland (also Pyrenees).
7. Inula salicina. W. Ireland (Scandinavia, Middle and South Europe).
8. Erica mediterranea. W. Ireland (W. France, Spain, Portugal).
9. ,, mackaiana (tetralix sub.-sp.) W. Ireland (Spain).
10. Arbutus unedo. S. W. Ireland (W. of France, Spain, Portugal and shores of Mediterranean).
11. Dabeocia polifolia. W. Ireland (W. of France, Spain and Portugal).
12. Pinguicula grandiflora. S. W. Ireland (Spain, Pyrenees, Alps of France and Switzerland).
13. Neotinea intacta. W. Ireland (S. France, Portugal, Spain, and shores of Mediterranean).
14. Spiranthes romanzoviana. S. W. Ireland (North America).
15. Sisyrinchium angustifolium. W. Ireland (North America, Arctic and Temp.).
16. Potamogeton lonchites. Ireland, Mr. Arthur Bennett informs me that this is certainly not British or European, but may possibly be identical with P. fluitans var. Americanus of the U. States.
17. Potamogeton kirkii (natans sub.-sp.). W. Ireland. (Arctic Europe?)
18. Eriocaulon septangulare. W. Ireland, Skye, Hebrides (North America).
19. Carex buxbaumii. N. E. Ireland, on an island in Lough Neagh (Arctic and Alpine Europe, North America).
20. Deyeuxia neglecta (var. Hookeri). On the shores and islands of Lough Neagh. (And in Germany, Arctic Europe, and North America.)
We find here nine south-west European species which probably had a wider range in mild preglacial times, and have been preserved in the south and west of Ireland owing to its milder climate. It must be remembered that during the height of the glacial epoch Ireland was continental, so that these plants may have followed the retreating ice to their present stations and survived the subsequent depression. This seems more probable than that so many species should have reached Ireland for the first time during the last union with the continent subsequent to the glacial epoch. The Arctic, Alpine, and American plants may all be examples of species which once had a wider range, and which, owing to the more favourable conditions, have continued to exist in Ireland while becoming extinct in the adjacent parts of Britain and Western Europe.