279. As with the horizontal areas of the water, so in like manner with its various depths or bathymetrical zones. The littoral zone (par. 267) of our own seas, for example, is characterised, according as the bottom may be rocky, sandy, or muddy, by such shell-fish as the periwinkle, limpet, mussel, cockle, and razor-shell; the laminarian by star-fishes, the common sea-urchin, tubularia, modiola, and pullastra; the coralline by the disappearance of the ordinary shore shells, and the abundance of buccinum, fusus, trochus, venus, pecten, and the like; and the coral zone by forms of star-fish, cidaris, and brachiopod mollusca that cannot exist in shallower waters. As to the extreme depths of ocean, they are generally presumed to be, like the extreme elevations of the land, altogether barren and lifeless. It is true that some recent deepsea soundings in the North Atlantic would seem to point to a different conclusion; but these, before final acceptance, require the confirmation of further and independent experiment.

280. It must ever be borne in mind, however, that, obvious as may be the influence of external conditions on the distribution of animal life, there is over and above them an aboriginal dispersion that science in its present position is unable to account for. Why, for instance, should the kangaroo and ornithorhynchus be restricted to Australia, the apteryx to New Zealand, the hippopotamus and giraffe to Africa, the camel to the Old World, and the llama to the New? Or why should the New World, notwithstanding its predominance in vegetation, be inferior in variety and intensity of animal life to the Old? But while these aboriginal differences exist and remain unaccounted for, the student should remember that distant regions, having nearly the same conditions, are peopled by what are termed representative species—that is, species not zoologically identical, but merely representing one another in the economy of nature, and fulfilling similar functions. Thus, the camel of the Old World is represented by the llama of the New; the lion and tiger of the Old World by the puma and jaguar of the New; the ostrich of Africa by the rhea of South America and the emu of Australia; and the crocodile of the Nile by the gavial of the Ganges and the alligator of the Amazon. It should also be borne in mind that local conditions of food, shelter, healthful position, freedom from enemies, and the like, are also operating causes in the distribution and dispersion of animal life; and that where these are wanting, whole families will shift ground, or altogether disappear, even when the great conditions of temperature remain unchanged.

281. As in the vegetable world, so also (though to a less extent) in the animal, certain species have a greater elasticity of

constitution, which enables them to subsist under a greater variety of conditions, and naturally, therefore, to enjoy a wider geographical range. Operating upon this principle, and a knowledge of climatology, man has been enabled to transfer from one region to another a considerable number of animals, either for the purposes of his convenience or luxury. All the domestic animals —horse, ass, ox, sheep, goat, pig, dog, cat, barn-fowl, &c.; many birds, prized for their beauty or song; and rats, mice, insects, and other creatures, considered as "pests and vermin," have accompanied him over the habitable globe. His efforts in this respectextirpating, transferring, and acclimatising-have been incessant; and thus creatures naturally of widely distant habitats have been, and are still being, brought together into one common area. In this way the domestic animals of the Old World have been transferred to the New, where they were unknown at the time of its discovery by Columbus; some of the New World fauna transferred to the Old; and not only the domestic animals, but the birds, fish, and even shell-fish of Europe, transported to Australasia, where, within little more than a century, their genera were totally unrepresented. But as in the vegetable world, so in the animal, there is a limit to this system of transference, and man best studies his own interest and the comfort of the lower animals by fostering them mainly within their own native habitats, and sharing in their larger produce through the more profitable method of commerce and exchange.

Interdependence of Plants and Animals.

282. As plants and animals are alike dependent on external conditions, so both are, to a certain extent, dependent on one another. Both, for example, are dependent on the atmosphere, yet the oxygen which the plant exhales is inhaled by the animal, and the carbonic acid exhaled by the animal is absorbed and assimilated by the plant. The plant, rooted in the soil and casting abroad its leaves and branches in the atmosphere, though seemingly deriving the main elements of its growth from inorganic sources, is nevertheless stimulated into life and exuberance by the presence of organic decay, and many of the lower fungusgrowths are found only where such decomposing matter is present. Herbivorous animals, as is well known, subsist directly upon plants, while the carnivorous prey upon the plant-feeders, and are thus also ultimately dependent on the vegetable world for their subsistence. Wherever vegetable life is varied and

luxuriant, there animal life is marked by a corresponding variety; hence the specific exuberance of the tropics compared with that of the colder latitudes. By the extirpation of certain plants, certain mammals, birds, and insects may be removed from a district; while, on the introduction of some new exotic, animals hitherto unknown in that locality usually make their appearance. Certain birds, for example, feed on certain insects, and these insects, again, find their chosen food in certain plants; remove the plants and you destroy the insects, and by the destruction of the insects you compel the birds to remove and find supplies in other habitats, or if these supplies cannot be found the birds are extirpated. The law of circulation and interdependence is complete, and no portion of the circle could be removed without a corresponding change in the characters of the vegetable and animal kingdoms.

283. Again, though most plants have the inherent power of dispersing their own seeds, and are aided in this by winds and water-currents, yet many depend upon birds and mammals for their wider dispersion and increase, just as many depend upon insects for the fertilisation of their flowers. This wider dispersion creates a new source of subsistence for the animals that feed upon them, and thus the increased area of the one supplies a wider range to the other. Further, as some animals are fitted by their organisation for an arboreal existence-some for life on the grassy plain, and others for the shrubby thicket-the destruction of the tree, the planting of the plain, or the clearing of the thicket, would necessarily involve the destruction of these special organisations. Still further, as some creatures are specially fitted to live on fruits, some on leaves, and others on roots, the disappearance of these specific supplies would necessarily be followed by the annihilation of the consumers. It is in this manner that plants and animals become co-dependent portions of one great vital plan, and that geographers, aware of these relations, can more intelligibly depict the aspects of nature by associating every fauna with its own appropriate and distinctive flora.

NOTE, RECAPITULATORY AND EXPLANATORY.

In the two preceding chapters attention has been directed to the vital aspects of the globe—that is, to its vegetable and animal life, and their distribution over the land and through the

waters. Leaving the nature and origin of Life to the biologist, it has been shown that plants and animals are dependent for their continuance on the external conditions by which they are surrounded, and that any change in these conditions would materially affect, if not destroy, their existence. For this reason, plants and animals have a definite distribution over the globe; heat, light, and moisture being the great regulators of the one; climate and food the governing conditions of the other. In this way vital variety and exuberance culminate within the tropics and decline as we proceed towards either pole-declining also in an analogous manner as we ascend from the level of the sea into the higher elevations of mountains. Each zone or belt of the earth has thus its own special flora and fauna—that is, is characterised by genera and species not naturally occurring in other regions. As marked variations occur within narrower limits than the torrid, temperate, and frigid zones, botanists subdivide the earth's surface into equatorial, tropical, sub-tropical, warmertemperate, colder-temperate, sub-arctic, arctic, and polar belts; and these, as might be expected, correspond with lines of temperature rather than with parallels of latitude. As in the land, so also in the waters, each zone of depth, from the shore seaward, has its characteristic forms-littoral, laminarian, coralline, and coral; the extreme abysses of ocean, like the extreme altitudes of land, being apparently barren and lifeless.

Beyond these main horizontal and vertical arrangements of life, there is also an aboriginal dispersion of certain races over certain areas which science cannot account for; hence the subdivision of the earth's surface into botanical and zoological "regions" and "provinces," each subdivision being characterised by its own typical forms. These provinces form the special study of the botanist and zoologist; and the question how far the plants and animals of one region can be profitably transferred to another, becomes one of prime economical importance. Understanding the geographical conditions under which plants and animals naturally occur, and knowing, moreover, the intimate dependence between the vegetable and animal kingdoms, man, in his own migrations over the globe, will be better able to determine what to cultivate and what to extirpate, what to attempt acclimatising and what to continue in their own native habitats. Understand

ing, moreover, the relations that subsist between fauna and flora, he will, as a geographer, be better enabled to draw that intelligible picture of external aspects which it is the grand province of his science to depict; and from a knowledge of these aspects be further enabled to indicate to the merchant and trader the

nature and amount of the products which each separate region can supply.

The student who would enter more fully into the distribution of vegetable and animal life may consult for the former such works as Balfour's 'Class-book of Botany,' Lindley's 'Vegetable Kingdom,' Schouw's 'Earth and Man,' Meyen's 'Botanical Geography,' and the various publications of Humboldt, Hooker, and others who have written on botany; and for the latter the letterpress of Johnston's 'Physical Atlas,' the 'Zoologies' of Cuvier, Milne-Edwards, Carpenter, &c.; and the various writings of Agassiz, Darwin, Edward Forbes, and the like, who have ventured on the higher questions of zoological geography.