suit the sunny flight of the eagle, the twilight mousing of the owl, the brousing of the ruminant, the nocturnal watch of the lion; the watery life of the whale, or the fish, where the different refractive power of the fluid in which they live is accompanied by modifications, not alike in each, but yet alike in the manifestation of intentional adaptation. Consider in the same point of view the large orbit of Ichthyosaurus with its broad circlet of sclerotic bones; or the reticulated lenses of the Trilobite; or finally, the black or red spots sensitive only to light in the Mollusca and Radiata; or scrutinize in the same way any other organ of sense, in the animals of every age, and inquire with the Psalmist: He who planted the ear, shall He not hear ? Life runs always the same course of growth, decay and death, in the individual-always the same course of renewal by offspring, after a definite mode which varies with the different kinds of living beings. One of these modes seems to deserve separation from the rest, under the title of fissiparism, because in it the individual seems divided, and so the number of individuals multiplied. This obtains among the Polyparian Zoophyta, and may be paralleled among many plants, and artificially exemplified by cuttings. 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 64° 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 England1. 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 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 1 Humboldt; Hooker, Indian Flora; Watson's British Plants; Balfour's Class Book; Somerville's Phys. Geography, Meyen, Botanical Geography. 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 expire1. 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. |