Physical geography. By Mary Somerville ...

CHAP. XXIII. BOTANICAL DISTRICTS. 329
whole families of plants being confined to one particular country,
or even to a very limited district, which, as far as we can judge,
might have grown equally well on many others. Latitude, elevation, soil, and climate are but secondary causes in the distribution
of the vegetable kingdom, and are totally inadequate to explain why
there are numerous distinct botanical districts in the continents and
islands, each of which has its own vegetation, whose limits are most
decided when they are separated by the ocean, mountain-chains,
sandy deserts, salt-plains, or internal seas. Each of these districts
is the focus of families and genera, some of which are found nowhere else, and some are common to others, but, with a very few
remarkable exceptions, the species of plants in each are entirely
different or representative.' This does not depend upon the dif1 M. de Candolle established 20 botanical regions, and Professor Schow
20; but Professor Martius, of Munich, has divided the vegetation of the
globe into 51 provinces, namely, 5 in Europe, 11 in Africa, 13 in Asia, 3
in New Holland, 4 in North and 8 in South America, besides Central
America, the Antilles, the Antarctic Lands, New Zealand, Van Diemen's
Land, New Guinea, and Polynesia. To these, other divisions might be
added, as the Galapagos, which is so strongly defined.
Baron Humboldt gives the following concise view of the distribution of
plants, both as to height and latitude: —
The equatorial zone is the region of palms and bnanans.
The tropical zone is the region of tree-ferns and figs.
The subtropical zone, that of myrtles and laurels.
The warm temperate zone, that of evergreen trees.
The cold temperate zone, that of European or deciduous trees.
The subarctic zone, that of pines.
The arctic zone, that of rhododendrons.
The polar zone, that of alpine plants.
lUpper Limit of Trees on Mffountains.- The upper limit of trees is distinguished by the Escallonhe, on the Andes of Quito, at the height of 11,500
feet above the level of the sea.
In tropical Mexico the upper limit of trees, at the height of 12,789 feet,
is distinguished by the Pinus occidentalis.
In the temperate zone the limit of trees is marked by the Quercus Semicarpifolia, at 11,500 feet, on the south side of the Himalaya, and by the
Betula Alba, on the north side, at the height of 14,000 feet; the same birch
forms the limit of the Caucasus, at the elevation of 6394 feet. On the
Pyrenees and Alps the limit is marked by the Conifers or pine tribe: on
the Pyreness by the Pinus uncinata, at the height of 10,870 feet; on the
south side of the Alps by the larch, at the elevation of 6700 feet; and by
the Pinus abies, at 5883 feet on the north.
In Lapland the Betula Alba forms the upper limit of trees, at the height
of only 1918 feet.
The upper Limit of Shrubs.-In the Andes of Quito the Bejarias are the
shrubs that attain the greatest height, and terminate at 13,420 feet above
the sea-level.
The juniper, Salix, and Ribes, or currant tribe, form the upper limit of
shrubs on the south side of the Himalaya, at the height of 11,500 feet.
The tama, or Genista versicolor, a species of broom, flourishes at the height
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