On the Origin and Distribution of the British Flora. 75
followed by one of great depression, reducing Great Britain
to an archipelago, in which only alpine plants would survive.
Then followed a second continental period, readmitting the
flora of Europe to the British Isles, but so rapidly succeeded
by depression to present levels that, reckoning the flowering
plants and ferns of Great Britain at 1425, only 970 had time
to reach Ireland.22
As we travel eastwards from the Mediterranean, through
the Levant, Caucasus, Persia, the Himalayas, China, and
Japan, we find the traces of the retreating American Miocene
flora more and more numerously. The fan-palm, the plane-
tree, and the walnut of the East, and the magnolias of the
Himalayas, China, and Japan, have the meaning of their
distribution still more strongly brought out by the discovery
of a tulip-tree in Central China.23
Mr. Bentham points out24 that the northern flora has
undergone a specialisation into three secondary floras, the
Arctic-Alpine, the Temperate, and the Mediterraneo-Caucasian.
The first of these, common in some degree to the Old and
New World, has been driven into every latitude, surviving on
the mountains when glacial gave way to warm conditions,
often on their southern slopes. The Temperate flora consists
largely of genera common to every longitude, easterly exten-
sions of American groups, such as the deciduous trees;
whilst the Mediterraneo-Caucasian, comprising six-sevenths
of the species of Europe, and bounded by the deserts of
Africa and Arabia, but having outliers on the mountains of
Tropical Africa, may represent the remnant of the flora of
Europe previous to the Arctic-Alpine and American Miocene
invasions.
Whilst we are not concerned with the Tropical flora, that
of the disconnected lands of the South has an important
bearing on our present subject. In the West of Europe is a
22 Hewett Watson, cited by Mr. Wallace, op. cit., p. 320.
23 Moore, 'Journal of Botany,' 1875, p. 225; Oliver, 'Natural History
Review,' 1862; and W. T. T. Dyer, article "Distribution," Encyclop.
Britan., 9th ed., vol. vii. (1877), p. 287.
24 Op. cit., summarised by Dyer, op. cit.