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But in general, there is the preparation of egg or seed, the development of these in connection with nutritive matter prepared in each, and the passage through several stages before the complete state of individual life is attained; then recurs the separation of parts impressed with the wonderful power, and subject to the inconceivable restraint, of so acting on and being acted on by the elementary powers around, as to grow, reproduce and decay, in forms and through periods corresponding to those appointed for their ancestral races since time began.

INFLUENCE OF CLIMATE ON THE DISTRIBUTION

OF LIFE.

Not the whole surface of the earth is occupied by living beings. Notwithstanding the perpetual struggle to diffuse their seed, plants do not cover all the regions of the land ; nor is the amazing fertility of many marine animals able to carry life into all parts of the sea. The geographical distribution of plants and animals, a subject of great richness and instruction, offers some general facts which must not be neglected in reasoning on the ancient forms of life which fill the stratified rocks.

Among the most influential of all the causes which limit the ranges of life is temperature. The annual mean temperature, the extremes of yearly and daily heat and cold, and the humidity of the atmosphere, in a good degree dependent on temperature, are conditions to which life in general, and special forms of life in particular, are adjusted.

For example, proceeding from the equator to the north, along the land in the new or the old world, we find the number of the forms of life continually grow less and less. According to an estimate of some date, if we count in the warm zones of Asia the plants which range northwards from the equator with a temperature of 80°, so as to reach the latitude where the annual mean temperature of 64° prevails, we shall find 4500 species ; between 64o and 48° the number is reduced to 1500, while between 32° and 0° the total sinks to 500, till in Walden Island, lat. 80% north, only ten species occur ; and finally, the whole series becomes extinct.

So if we estimate the land mammalia of the Tropical zone at 800 species, the proportionate number for the Temperate zone is 200, and for the Polar zone twenty. But the reverse holds in regard to the Cetacea, which increase towards the Polar oceans.

It may be further observed that the earth seems to have the two extremes of life, the hot extreme in the deserts of Africa, the cold extreme toward either pole, and toward the summits of high mountains. The American Flora is richer than that of the old world. If 13000 be the number of phanerogamous plants in Tropical America, 4000 may be taken to represent the flora in the Temperate zone. If 2000 plants can be collected within a radius of ten miles in India, about 500 can be found in an equal space of the surface of England".

In Mr Watson's interesting work on the Geographical Distribution of British Plants, we find the Highland plants of Scotland grouped in three divisions according to elevation-1000 to 2000 ft., 2000 to 3000 ft. and 3000 to 4000 ft. above the sea. The numbers are :-

273 species—183 species—85 species
48 orders-38 orders—25 orders,

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shewing clearly the reduction of vegetative energy with increased elevation.

Equally positive is the limitation fixed by climate upon the geographical range of the different natural groups of plants. Proceeding northward from the equator we pass through the fruitful equatorial region of the bananas and palms, the tropical zone of arborescent ferns and figs, the sub-tropical zone of myrtles and laurels, the warm temperate zone of evergreen trees, the cold temperate zone of deciduous trees, the sub-arctic zone of pines, the arctic zone of rhododendra, and finally, the polar zone where the last relics of vegetable life expire?.

2 Humboldt; Hooker, Indian Flora; Watson's British Plants; Balfour's Class Book; Somerville's Phys. Geography, Meyen, Botanical Geography.

The same succession of plants is found in ascending the lofty mountains of the equatorial zone, as Chimborazo in South America, and Popocatepetl in North America, whose summits have the polar climates, and their slopes the plants and animals of different latitudes. The ranges are equally definite on Etna, but there the palm zone is only at the very base; and on Mont Blanc, which has none of the lower and

warmer zones.

In a diagram, Fig. 1, the distribution in latitude of several races of mammalia is traced by lines. If we regard the two halves of the circle divided by the central meridian as representing the old and new world, we shall find in each the same laws prevail for the same groups; though none of the species are the same in the two regions, except toward the north pole, where there is almost a connection of the two countries by islands and ice.

Thus the Platyrhine Monkeys of the new world are balanced by the Catarhine Quadrumana of the old world; the Feline races of Asia and Africa, the

1 Humboldt.

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6401

64°

0

80°

OLD WORLD.

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