“Baltic” flora on the north and the “Mediterranean” flora on the south, but chiefly to the presence on their heights of a third flora which has but little in common with either of the others. The stronghold of this last, the distinctively “Alpine” flora, is the region above the tree-limit. Its closest relationship is with the flora of the Pyrenees; but an alpine flora is characteristic of all the lofty mountains of central Europe. According to J. Ball, 2010 well-marked species of flowering plants occur within the limits of the Alps. If now we confine our attention to the alpine and higher regions of the Alps and exclude from our list all those plants which, however abundant in these regions, are not less so in the adjacent lowlands, we have left some 700 species (693, according to Dr Christ). We must observe, as regards the plants of the lower alpine region, that it is the actual presence of a forest vegetation, rather than the theoretical tree-limit, which affects their vertical distribution; so that, e.g. they overflow into the extensive clearings made by man in the primeval mountain forests. Indeed, an analysis of the composition of the alpine flora as a whole leads to the conclusion that the chief bond of union between its members consists in the treeless character of their habitat.
We may broadly distinguish two main geographical elements in the alpine flora, namely, the northern element and the endemic element. This division (which is not, however, strictly exhaustive) directs special attention to what is undoubtedly the most striking feature of the flora—namely, that of its 693 species no less than 271 reappear in the extreme north. This relation of the arctic to the alpine flora is all the more remarkable in view of the very important differences between the arctic and alpine climates. The following circumpolar species are common, and widely diffused throughout the whole of the Alps: Silene acaulis, Dryas octopetala, Saxifraga oppositifolia, S. aizoides, S. stellaris, Erigeron alpinus, Azalea procumbens, Myosotis alpestris, Polygonum viviparum, Salix retusa, S. herbacea, Phleum alpinum, Juniperus nana. The proportion of northern forms, as regards both species and individuals, increases as we ascend to the higher regions. In the highest vegetation-zone, the snow-region—i.e. on islands of rock above the snow-line—they attain to an equality with the endemic forms. As examples of northern flowers which are characteristic of the snow-region, we may mention Silene acaulis, Eritrichium nanum and Arenaria ciliata. On the other hand, typical endemic species of this highest zone are Androsace helvetica, A. glacialis, Petrocallis pyrenaica and Cherleria sedoides. All the plants just named, we may observe, are “cushion-plants.” Their compact, moss-like growth and general structural peculiarities are not an expression of mutual affinity, but are in adaptation to the combined cold and dryness of their habitat. It is noteworthy that among the northern plants of the alpine zone, in the narrower sense of the term (i.e. of the region between the tree-limit and the snow-line), there is a marked predominance of species that affect moist localities; and conversely, the majority of alpine flowers of wet habitat are found also in the north. For example, in the genus Primula, a highly characteristic genus of the alpine flora, whose members are among the most striking ornaments of the rocks, the single northern species, P. farinosa, grows only in marshy meadows. On the whole, then, adaptation to cold and wet is the note of the northern element.
As for the explanation of the community between the alpine and arctic floras, all authorities are agreed that the key to the problem is furnished by the occurrence of the glacial period. In the ice-free belt, between the northern ice-sheet and the vastly extended glaciers of the Alps, the two floras must have found a common refuge and congenial conditions of existence; and this view is confirmed by direct palaeontological evidence. With the return of a milder climate, the so-called northern forms of the present alpine flora were split in two, one portion following close on the northern ice in its gradual retreat to the Arctic, the other following the shrinking glaciers till the plants were able to establish (or re-establish) themselves on the slopes of the Alps. The same explanation covers the case of the similarity of the flora (not merely as regards the northern element) on all the high mountains of central Europe. So much seems to be beyond reasonable doubt. But at this point disagreement begins between the most eminent writers on the subject. While some (e.g. Sir J. D. Hooker, Heer) regard the Arctic, and some (e.g. Wettstein) the Alps, as the original home of at least the bulk of the “northern” element, others (e.g. Ball, Christ) locate this in the highlands of temperate Asia. For it is a remarkable fact that, of the 230 northern species which are most typical of the far north, 182 are found also in the Altai (taking this as a collective name for the mountains that form the southern boundary of Siberia). In any case, however, the migration of these plants to the Alps must for the most part have taken place via the Arctic. The possibility of any extensive east to west migration having taken place direct from the Altai to the Alps seems excluded by the fact that 50% of the arctico-altaic alpine plants are absent from the Caucasus. A score of species, it is true—not such a number, be it observed, as was formerly supposed—are common to the Alps and Altai, but absent from the Arctic. But the species composing this Altaic element are not so numerous as the arctico-alpine species that are absent from the Altai. On the whole, a common origin in the north for at least the arctico-altaic group of alpine plants seems to be the most reasonable hypothesis.
Side by side with the northern element (which in some respects, we may observe to point the contrast, would be better named the tundra-element) we find a group of species usually spoken of as the xerothermic or meridional element. These do not, however, form an “element,” in the strict geographical sense in which this term is otherwise used here. They are those species which, on general phyto-geographical grounds, must be regarded as having originated under steppe-like conditions. Their affinities are chiefly, though not exclusively, with the present Mediterranean flora—about fifty are of presumably Mediterranean origin—and a large proportion of them are restricted to the southern slopes of the Alps. The following, however, among others, are distributed throughout the whole, or a great part, of the range: Colchicum alpinum, Crocus vernus, Orchis globosa, Petrocallis pyrenaica, Astragalus depressus, A. aristatus, Oxytropis Halleri, Eryngium alpinum, Erica carnea, Linaria alpina, Globularia nudicaulis, G. cordifolia, Leontopodium alpinum. The last named (the well-known “edelweiss”) is at the present day characteristic of the Siberian steppes. The presence of these plants among the alpine flora is traceable to the steppe-like conditions which prevailed in central Europe both during the warmer inter-glacial periods and (probably) for a time after the close of the ice-age. Subsequently, as the climate of the plains assumed a colder and more humid character, they retired before the invading forests to the high mountains. Here, in the intenser insolation which they enjoy on the alpine slopes, they seem to find a compensation for the drawbacks incidental to the altitude of their present station.
As regards now the endemic element as a whole, the question as to the time and place of its origin is of a highly complicated and controversial nature. The question, too, in the case of this element, is necessarily of genetic rather than purely geographical scope. It must suffice to say that the weight of scientific opinion inclines to the view that at least the majority of endemic species are of pre-glacial origin, and are either strictly indigenous or products of the neighbouring lowlands. About 40% of the endemic element in the alpine flora are endemic also in the narrower sense, i.e. they are confined to the Alps. Many of them are restricted to some one small portion of the chain; these occur chiefly in the southern and eastern Alps. It is an interesting fact that the centrally situated Bernese Alps produce hardly a single peculiar species. The greater richness of certain districts in the matter of species is partly due to the variety of soils encountered therein; but in part may be explained by the fact that these districts were the first to be freed from the ice-sheet at the end of the glacial period.
The following is a list of the most thoroughly characteristic alpine plants—all of them ipso facto members of the endemic element—which are at once peculiar to the Alps (or practically
