288 DISTRIBUTION [VEGETABLE. the more southern divisions of the Northern flora, so we may infer that towards the close of the Tertiary epoch the continuous circumpolar land was covered with a vegetation also largely composed of identical plants but adapted to a wanner climate. As the climate became less warm there would commence a migration southwards, which would result in the modified descendants of these plants being now blended with the vegetation of Central Europe and the United States. As the glacial period gradually advanced, " the tropical plants and animals will have retreated from both sides towards the equator, followed in the rear by the temperate productions, and these by the arctic." 1 When the climate of the earth again ameliorated, the migration took place in the reverse direction, -and in this way mountain ranges became the havens of refuge of fragments of the original arctic floras which were exter minated on the lowlands. Even the equatorial region ceased to be a barrier during the glacial period, and to migration at that time must be attributed the survival of arctic forms in the south temperate zone. The southern migration of the Arctic flora does not appear to have taken place in one continuous wave. Thus, as Bentham points out, 2 " many facts showed separate communications between the north and each of the three chains of the Pyrenees, the Alps, and the Himalayas, whilst these three gave little evidence of any lateral communication of their respective alpine vegetations." The fact that the migration southwards and remigration northwards of the Arctic flora took place along parallels of longitude, accounts for some of its existing peculiarities. Hooker explains in this way the comparative poverty of the Greenland flora. 3 "If it be granted that the polar area was once occupied by the Scandinavian flora, and that the cold of the glacial epoch did drive this vegetation southwards, it is evident that the Greenland indi viduals, from being confined to a peninsula, would be exposed to very different conditions to those of the great continents. In Green land many species would, as it were, be driven into the sea, that is exterminated ; and the survivors would be confined to the southern portion of the peninsula, and not there being brought into competi tion with other types, there could be no struggle for life amongst their progeny, and consequently no selection of better adapted varieties. On the return of heat, these survivors would simply travel northwards unaccompanied, by the plants of any other country. " In Arctic America and Asia, on the other hand, where there was a free southern extension and dilatation of land for the same Scandinavian plants to occupy, these would multiply enormously in individuals, branching off into varieties and sub-species, and occupy a larger area the further south they were driven ; and none need be altogether lost in the southern migration over plains, though many would in the struggle that ensued when they reached the mountains of those continents and were brought into competi tion with the alpine plants, which the same cold had caused to descend to the plains. Hence, on the return of warmth, many more Scandinavian species would return to Arctic America and Asia than survived in Greenland ; some would be changed in form, because only the favoured varieties could have survived the struggle ; some of the alpine Siberian and Kocky Mountain species would accom pany them to the arctic zone ; while many arctic species would ascend those mountayis, accompanying the alpine species in their reascent." The Arctic-alpine flora is obviously in its present condi tion a composite one. Portions of the Northern flora, probably originally very distinctly characterized, became adapted to the peculiar physical conditions of high moun tain ranges and of the extreme north. The gradual deterioration of the climate brought the alpine flora, to the lowlands and the arctic flora southwards till they intermingled. When they again returned to their original territories they were so far changed that each gave the other some new members, while both had experienced many losses. 1 Darwin, Origin of Species, 4th ed. p. 447. 2 Presidential address, 1869, p. 21. 3 Hooker, 1. c. p. 254. A. de Candolle has very ingeniously applied the general principles laid down above to the detailed explanation of the distribution of the flora of the Alps themselves. The following is a brief summary of his conclusions : 4 The valleys and groups of mountains which have at present a _ maximum of rare species and the most varied flora belong to districts on which the glaciers disappeared earliest. On the other hand, where the duration of snows and glaciers has been most pro- .longed, the existing flora is poor. From a variety of causes which A. de Candolle enumerates, it seems probable that the southern and eastern glaciers of the Alps were of smaller extent than the northern, and would consequently be the soonest to retreat. We have consequently the curious fact that some of the most ancient fragments of the alpine flora are now only to be found 011 the southern slopes of the Alps. This is the case with species of Primula, Pedicularis, and Oxytropis, which exist neither in the interior of Switzerland, nor in the north of Europe. But it is easy to see that, like the other members of this flora, they were driven south during the glacial period, returning as the mountains reappeared from underneath their snowy covering, while on the northern side they were in great measure exterminated. A. de Candolle points out as a fact in further confirmation that the Alpine species of Campanula, peculiar to Mont Cenis and the Simplon and neighbouring valleys, are not related to the Arctic species, but find their nearest allies in Greece, Asia Minor, and the Himalaya. A further indication of the great antiquity of the Arctic- alpine flora is afforded by the fact of its absence in the comparatively modern volcanic mountains of France. " The Monts d Or and Cantal, at an elevation of 6000 feet, offer scarcely any of those alpine and sub-alpine plants which abound at the same, or lower elevations in the Pyrenees on the one side, and in the Alps on the other, as well as in the British and Scandinavian mountains to the north." 5 Hooker, however, points out that the absence of the alpine-arctic flora in Auvergne may be due to severe glaciation rather than to its absence (see Nature, Nov. 11, 1875, pp. 31, 32). 2. The Intermediate or Temperate flora is best described in the words of Bentham as "A mongrel vegetation of mixed origin, including a large pro portion of species of the most extended geographical range, with a very few local ones, and those chiefly in the extreme west. The majority, whether trees, shrubs, or herbs, are plants of compara tively rapid growth, very prolific, endowed with great facilities for dispersion, and constitutions capable of adapting themselves to a great variety of physical and climatological conditions. They are great travellers, and soon take possession of any district left denuded by the abandonment of cultivation. To the great majority of them no primeval antiquity can be ascribed in Central or Western Europe ; 6 they appear to have come from the east, a considerable number perhaps from Western Asia, where their types appear to be more varied, but many also must have made half the tour of the globe. Large American genera have sent out offsets into Eastern Asia, which gradually diminishing in number of species, and some times slightly modifying their character, have spread over the whole of Asia, and invaded almost every part of Europe. These plants are, moreover, generally continuous, that is, interrupted only by intervals which under present condition they have means of cross ing ; and they are abundant in individuals, ascending in latitude and elevation, or descending to the south, until checked in their career by competing species, better enabled to endure the increas ing rigour or the searching drought of the respective climates. Many of them will even assume slight modifications suited to their exceptional circumstances, and it is then as difficult to separate, them from the genuine northern or southern floras as in many cases to give plausible grounds for establishing the precise origin of individual species." 7 The peat deposits of Denmark tell an unmistakable tale of the gradual advance of successive waves of vegetation from the south-east. The Scotch fir was once abundant within 4 These are given in greater fulness in Nature, April 27, 1876, p. 516. 5 Bentham, Nat. Hist. Rev., 1SC4, p. 370. 6 Martins, however, considers that many of the plants of the existing south of Europe flora are of great antiquity in their present situations ; thus the Oleandei (Nerium Oleander) has been found in deposits from the Eocene upwards (Mem. de I Acad. d. So. de Mont- pettier, ix. p. 95).
7 Nat. Hist. Rev., 1864. pp. 370, 971.