now unrepresented anywhere but in South Africa.” This flora
extends from Ireland to the Canaries and reappears on the highlands
of Angola. On the eastern side the southern flora finds representatives
in Abyssinia, including Protea, and on the mountains of equatorial
Africa, Calodendron capense occurring on Kilimanjaro. This
is the most northern representative of the Rutaceous Diosmeae,
which are replaced in Australia by the Boronieae. The Proteaceous
genus, Faurea, occurs in Angola and Madagascar. The characteristic
genus Pelargonium has a few Mediterranean representatives,
and one even occurs in Asia Minor. In all these cases it is a nice
question whether we are tracing an ascending or descending stream.
Darwin thought that the migration southwards would always be
preponderant (Origin of Species, 5th ed., 458). Other characteristic
features of the flora are the abundance of Compositae, Asclepiadeae,
and petaloid Monocotyledons generally, but especially Orchideae
(terrestrial species predominating) and Iridaceae. There is a marked
tendency towards a succulent habit. The nearly related Ficoideae
replace the new-world Cactaceae, but the habit of the latter is simulated
by fleshy Euphorbias and Asclepiads, just as that of Agave is
by the liliaceous Aloe. South Africa has only two palms (Phoenix
and Hyphaene). In the Gnetaceous Welwitschia it possesses a
vegetable type whose extraordinary peculiarities make it seem
amongst contemporary vegetation much as some strange and extinct
animal form would if suddenly endowed with life. Conifers are
scantily represented by Callitris and Podocarpus, which is common to
all three sub-regions; and Cycads by the endemic Encephalartos
and Stangeria.
2. The Australian sub-region consists of Australia, Tasmania, New Caledonia and New Zealand, and, though partly lying within the tropic is most naturally treated as a whole. They are linked together by the presence of Proteaceae and of Epacrideae, which take the place of the nearly allied heaths in South Africa. The most dominant order in Australia is Leguminosae, including the acacias with leaf-like phyllodes (absent in New Zealand). Myrtaceae comes next with Eucalyptus, which forms three-fourths of the forests, and Melaleuea; both are absent from New Caledonia and New Zealand; a few species of the former extend to New Guinea and one of the latter to Malaya. Cupuliferae are absent except Fagus in Australia and New Zealand. The so-called “oaks” of Australia are Casuarina, which also occurs in New Caledonia, but is wanting in New Zealand. The giant rushes Xanthorrhoea and Kingia are peculiar to Australia. Palms are poorly represented in the sub-region and are of an Indo-Malayan type. Amongst Conifers, Podocarpus is found throughout, Agathis is common to Australia, New Zealand and New Caledonia; Araucaria to the first and last. Of Cycads, Australia and New Caledonia have Cycas, and the former the endemic Macrozamia and Bowenia. The Australian land-surface must be of great antiquity, possibly Jurassic, and its isolation scarcely less ancient. In Lower Eocene times its flora appears to have been distinctly related to the existing one. Little confidence can, however, be placed in the identification of Proteaceous or, indeed, of any distinctively Australian plants in Tertiary deposits in the northern hemisphere. The Australian flora has extensions at high levels in the tropics; such exists on Kinabalu in Borneo under the equator. The Proteaceous genus Helicia reaches as far north as China, but whether it is starting or returning must as in other cases be left an open question.
While the flora of New Caledonia is rich in species (3000), that of New Zealand is poor (1400). While so many conspicuous Australian elements are wanting in New Zealand, one-eighth of its flora belongs to South American genera. Especially noteworthy are the Andine Acaena, Gunnera, Fuchsia and Calceolaria. By the same path it has received a remarkable contribution from the North Temperate region; such familiar genera as Ranunculus, Epilobium and Veronica form more than 9% of the flowering plants. And it is interesting to note that while the tropical forms of Quercus failed to reach Australia from Malaya, the temperate Fagus crept in by a back door. Three-quarters of the native species are endemic; they seem, however, to be quite unable to resist the invasion of new-comers, and already 600 plants of foreign origin have succeeded in establishing themselves.
3. The Andine sub-region extends from Peru to the Argentine and follows roughly the watershed of the Amazon. In the New World, as already explained, the path of communication between the northern and southern hemispheres has always been more or less open, and the temperate flora of southern America does not exhibit the isolation characteristic of the southern region of the Old World. Taking, however, the Andean flora as typical, it contains a very marked endemic element; Ball finds that half the genera and four-fifths of the species are limited to it; on the other hand, that half the species of Gamopetalae belong to cosmopolitan genera such as Valeriana, Gentiana, Bartsia and Gnaphalium. The relation to the other sub-regions is slight. Ericeae are wholly absent, and it has but a single Epacrid in Fuegia. Proteaceae are more marked in Guevina and Embothrium. Of Restiaceae, a single Leptocarpus has been found in Chile. On the other hand, it is the headquarters of Mutisiaceae, represented in South Africa by such genera as Oldenburgia and Gerbera and by Trichocline in Australia. Tropaeolum takes the place of the nearly allied South African Pelargonium. There has been an interchange between it and the Mexico-American sub-region, but as usual the northern has been preponderant. Prosopis extends to the Argentine; other characteristic genera are Oenothera, Godetia, Collomia, Heliotropium and Eritrichium. In the ascending stream may be mentioned—Larrea, a small genus of Zygophylleae with headquarters in Paraguay and Chile, of which one species, L. mexicana, is the creosote plant of the Colorado desert, where it forms dense scrub; Acaena; the Loasaceae, of which Mentzelia reaches North America, Petunia and Lippia. Compositae compose a quarter of the Andean flora, which is a greater proportion than in any in the world. Baccharis, with some 250 species, ranges over the whole continent from the Straits of Magellan and, with seven species, to California. Melastomaceae, copiously represented in tropical America, are more feebly so in Peru and wholly wanting in Chile. A few Cactaceae extend to Chile. Of Cupuliferae, Quercus in three species only reaches Colombia, but Fagus, with only a single one in North America, is represented by several from Chile southwards and thence extends to New Zealand and Tasmania. The Magnoliaceous genus Drimys, with a single species in the new world, follows the same track. Bromeliaceae are represented by Rhodostachys and the temperate Puya. Palms as usual are few and not nearly related. Wettinia occurs in Peru, Trithrinax in Chile with the monotypic Jubaea, Juania, also monotypic, is confined to Juan Fernandez. Amongst Coniferae Podocarpus is common to this and preceding sub-regions; Libocedrus extends from California to New Zealand and New Caledonia; Fitzroya is found in Chile and Tasmania; and Araucaria in its most familiar species occurs in Chile.
4. The Antarctic-Alpine region is the complement of the Arctic-Alpine, but unlike the latter, its scattered distribution over numerous isolated points of land, remote from great continental areas, from which, during migrations like those attending the glacial period in the northern hemisphere, it could have been recruited, at once accounts for its limited number of species and their contracted range in the world. On the whole, it consists of local species of some widely distributed northern genera, such as Carex, Poa, Ranunculus, &c., with alpine types of strictly south temperate genera, characteristic of the separate localities. The monotypic Pringlea antiscorbutica, the “Kerguelen Island cabbage,” has no near ally in the southern hemisphere, but is closely related to the northern Cochlearia.
Such a summary of the salient facts in the geographical distribution of plants sufficiently indicates the tangled fabric of the earth's existing floral covering. Its complexity reflects the corresponding intricacy of geographical and geological evolution.
If the surface of the globe had been symmetrically divided into sea and land, and these had been distributed in bands bounded by parallels of latitude, the character of vegetation would depend on temperature alone; and as regards its aggregate mass, we should find it attaining its maximum at the equator and sinking to its minimum at the poles. Under such circumstances the earth's vegetation would be very different from what it is, and the study of plant distribution would be a simple affair.
It is true that the earth's physical geography presents certain broad features to which plants are adapted. But within these there is the greatest local diversity of moisture, elevation and isolation. Plants can only exist, as Darwin has said, where they must, not where they can. New Zealand was poorly stocked with a weak flora; the more robust and aggressive one of the north temperate region was ready at any moment to invade it, but was held back by physical barriers which human aid has alone enabled it to surpass.
Palaeontological evidence conclusively proves that the surface of the earth has been successively occupied by vegetative forms of increasing complexity, rising from the simplest algae to the most highly organized flowering plant. We find the ultimate explanation of this in the facts that all organisms vary, and that their variations are inherited and, if useful, perpetuated. Structural complexity is brought about by the superposition of new variations on preceding ones. Continued existence implies perpetual adaptation to new conditions, and, as the adjustment becomes more refined, the corresponding structural organization becomes more elaborate. Inheritance preserves what exists, and this can only be modified and added to. Thus Asclepiadeae and Orchideae owe their extraordinary floral complexity to adaptation to insect fertilization.
All organisms, then, are closely adapted to their surroundings. If these change, as we know they have changed, the organisms must change too, or perish. In some cases they survive by migration, but this is often prohibited by physical barriers.
