floating on the surface of stagnant water consists frequently of the shields of oxide of iron belonging to these animals. Ehrenberg has obtained several pounds weight of the silicious shields of infusoria, which he reared. In case tripoli, or "rotten stone," (which is a mass of fossil infusorial shells,) should become scarce, he proposes to supply the market from this source.

83. The rapidity of their multiplication is most astonishing, one individual of the Hydatina senta having been known to increase in ten days to 1,000,000; in eleven days to 4,000,000; and in twelve days to 16,000,000. Of another species, Ehrenberg says, one individual is capable of becoming 170,000,000,000,000 in four days. This rapid multiplication is effected by eggs, buds, and spontaneous division into two or more parts, each one of which very soon becomes a perfect animalcule; this also accounts for their wide diffusion, and sudden appearance in countless numbers. They are found in the waters, upon the land, and in the fluids of living, healthy plants and animals. Only those, however, which have hard shells, leave any trace of their existence after death. Ehrenberg has described about one thousand living species.

Snow is sometimes found in New Shetland and on the Alps, of a red color, and occasionally green. This is due to an admixture of

an infinite number of microscopic plants of low organizationmany of them of the tribe of Algæ. They

Fig. 45.

Infusoria in Snow.

are of globular form, celluler structure, and from one onethousandth to one three-thousandth of an inch in diameter.

Their liquid portions contain myriads of animalcules. Fig. 45 exhibits a magnified view of these singular beings. Their nature is adapted to a very low temperature, so that they can not bear a temperature above the freezing point of water, but when the snow melts they die.

84. Molluscous animals, by furnishing extensive accumulations of shells, essentially modify the structure of the surface of the earth. Shell-beds are found beneath the waters, and upon the shores of the ocean, lakes and ponds, producing beds of shell marl. Some shell-beds consist of fragments of numerous species mingled indiscriminately, broken and drifted by the waves and currents; while others are made up of a single species that lived in community, as the oyster, which in some places covers the bottom of the ocean, excluding all other genera, several miles in extent. Some species live only in the mud, while others seek sand for their habitations. Certain species live only attached to the shores, others in shallow water, and others still in water varying in depth from one hundred to one thousand feet. The number of families is much the greatest in shallow water, decreasing as we descend, and ceasing entirely in very deep water. Temperature, nature of the bottom, amount of light and food, determine the residence of each species. After the death of the animals, their shells, protected from decay, constitute in some instances a large part of the mineral bed forming by deposits from the water in which they lived.

85. The remains of fishes, reptiles, quadrupeds, birds, insects, and of men also, are entombed in deposits now forming, but in much smaller quantities, than those of corals, infusoria and shell-fish. Extraordinary occurrences, as the engulfment of cities by earthquakes, the destruction

of immense shoals of fishes by submarine volcanic agency, the overwhelming of herds of cattle by sudden inundations, and the drowning of clouds of locusts, produce accumulations of their remains, highly indicative of the state of the world at the time in which they lived, just as the fossils of the rocks, formed many ages since, are characteristic of the periods of their formation. But the bodies of most land animals, even the hardest portions of them—their skeletons-undergo decomposition upon dry land, and leave no trace of their forms.

86. The number of different kinds-species-of living plants and animals is very great. Eighty thousand species of plants have been described by botanists, and the entire number undoubtedly exceeds one hundred thousand. Professor Agassiz estimates the number of living species of animals at two hundred and fifty thousand; mammaliathose which suckle their young, two thousand; birds, six thousand; reptiles, two thousand; fishes, ten thousand; mollusks-shell-fishes, fifteen thousand; insects and crustaceous animals, one hundred thousand; and the star-fishes, coral polyps, &c., ten thousand. While individuals of each of the species are constantly dying, the species is perpetuated through centuries. Some species, however, have become extinct, within the observation of man. We have no proof of the introduction of a species since the creation of the human race.

87. The distribution of plants and animals upon the surface of the earth is very unequal, being influenced by the amount of heat, light, and moisture. Each geographical or climatal region has its own species; which, in the case of plants, constitute its Flora, and of animals, its Fauna. There are three great climatal regions, the arctic, temperate

and tropical. The vegetation of the arctic region is confined to mosses, lichens, and a few trees of stunted growth. The Flora of the temperate region embraces the nutritious grains and fruits, with lofty trees of dense fibre, durable and strong —the pine, oak, and cedar. The tropical region greatly excels the others, in the variety and luxuriance of its productions. The plants of different continents in the same latitude are quite unlike; those of Africa, for example, bearing little resemblance to those of South America or New Holland on the same parallels of latitude, each having been created in its own station.

88. The Faunas also are capable of distribution into three principal divisions, in accordance with climate, viz., the arctic, the temperate, and the tropical faunas. The plants and animals found at high elevations on mountains within the torrid zone, resemble those of colder latitudes.

The principal characteristic of the arctic fauna is its uniformity, embracing few species, but very great numbers of individuals in each species. The same animals are found in it on the three continents, America, Europe and Asia. Some large quadrupeds belong to this fauna, as the moose, the white bear, reindeer and musk-ox. Whales and seals abound in the polar seas, together with star-fishes, jellyfishes and small crustaceous animals, upon which the whale principally subsists: but very few polyps, and none which secrete coral are found in these seas. Very few insects live in this zone, and no reptiles. The color of the animals of the arctic fauna is frequently white, as shown by the white bear, the white fox, and the ermine, and when of other hues is not brilliant.

While the number of individuals of the temperate fauna is no greater than that of the arctic the number of species

is much greater and more varied. Very numerous orders and genera of animals, with strong contrasts of form and color, are here presented. The members of this fauna on different continents are similar: some of the families, genera, and a few species are identical. The arctic and temperate fauna are not separated from each other by any very sensible limit, but gradually pass into each other; a few species of animals range through the entire extent of both of them, as the musk-rat, the ermine and the European field-mouse.

The predominant feature of the tropical fauna, is its great variety of animals with coverings of brilliant hues. Its members on different continents are quite unlike each other; they are, however, more nearly allied to each other than to the members of the other faunas.

15. Besides faunas separated from each other by difference of climate, we have them more or less distinctly limited by geographical features. The interposition of mountain chains, deserts and seas separate faunas in the same latitude. The animals of the prairies of America, the steppes of Asia, and the deserts of Africa, are peculiar to those localities. The fauna of Oregon and California is said to be more unlike that of New England, than the European fauna is. Marine animals are distributed in the same way, into local faunas. The codfish does not wander far from the Newfoundland Banks. The fishes of the coast of South Carolina are different from those of the West Indies. Faunas that differ much are frequently found near each other, while similar faunas are oftentimes widely separated. The range of a species is not affected by its powers of locomotion. The reindeer is no more apt to transcend the limits assigned it, than is the oyster. The distribution