not found in neighboring streams. The garpikes (Lep: dosteus) of the American rivers afford a striking example of this kind.

409. A very influential cause in the distribution of aquatic animals is the depth of the water; so that several zoölogical zones, receding from the shore, may be defined, according to the depth of water; much in the same manner as we mark different zones at different elevations in ascending mountains, (398.) The Mollusks, and even the Fishes found near the shore in shallow water, differ, in general, from those living at the depth of twenty or thirty feet, and these again are found to be different from those which are met with at a greater depth. Their coloring, in particular, var.es, according to the quantity of light they receive, as has also been shown to be the case with the marine plants.

410. It is sometimes the case that one or more animals are found upon a certain chain of mountains, and not elsewhere; as, for instance, the Mountain Sheep (Ovis montana) upon the Rocky Mountains, or the Chamois and the Ibex upon the Alps. The same is also the case on some of the wide plains or prairies. This, however, does not entitle such regions to be considered as having an independent fauna, any more than a lake is to be regarded as having a peculiar fauna, exclusive of the animals of the surrounding country, merely because some of the species found in the lake may not ascend the rivers emptying into it. It is only when the whole group of animals inhabiting such a region has such peculiarities as to give it a distinct character, when contrasted with animals found in surrounding regions, that it is to be regarded as a separate fauna. Such, for example, is the fauna of the great steppe, or plain of Gobi, in Asia; and such indeed that of the chain of the Rocky Mountains may prove to be, when the animals inhabiting them shall be better known.

411. The migration of animals might at first seem to present a serious difficulty in determining the character or the limits of a fauna; but this difficulty ceases, if we regard the country of an animal to be the place where it makes its habitual abode. As to Birds, which of all animals wander farthest, it may be laid down as a rule, that they belong to the zone in which they breed. Thus, the gulls, many of the ducks, mergansers, and divers, belong to the boreal regions, though they pass a portion of the year with us. On the other hand, the swallows and martins, and many of the gallinaceous birds belong to the temperate faunas, notwithstanding their migration during winter to the confines of the torrid zone. This rule does not apply to the fishes who annually leave their proper home, and migrate to a distant region merely for the purpose of spawning. The Salmon. for example, comes down from the North, to spawn on the coast of Maine and Nova Scotia.

412. Few of the Mammals, and these mostly of the tribe of Rodents, make extensive migrations. Among the most remarkable of these are the Kamtschatka rats. In Spring they direct their course westward, in immense troops; and, after a very long journey, return again in Autumn to their quarters, where their approach is anxiously awaited by the hunters, on account of the fine furs to be obtained from the numerous carnivora which always follow in their train. The migrations of the Lemmings are marked by the devastations they commit along their course, as they come down from the borders of the Frozen Ocean to the valleys of Lapland and Norway; but their migrations are not period. ical.

17

SECTION II

DISTRIBUTION OF THE FAUNAS

413. We have stated that all the faunas of the globe may be divided into three groups, corresponding to as many great climatal divisions, namely, the glacial or arctic, the temperate and the tropical faunas. These three divisions appertain to both hemispheres, as we recede from the equator towards the north or south poles. It will hereafter be shown that the ropical and temperate faunas may be again divided into several zoological provinces, depending on longitude or on the peculiar configuration of the continents.

414. No continent is better calculated to give a correct idea of distribution into faunas, as determined by climate, than the continent of America; extending as it does across both hemispheres, and embracing all latitudes, so that all climates are represented upon it, as shown by the chart on the following page.

415. Let a traveller embark at Iceland, which is situated on the borders of the polar circle, with a view to observe, in a zoological aspect, the principal points along the eastern shore of America. The result of his observation will be very much as follows. Along the coast of Greenland and Iceland, and also along Baffin's Bay, he will meet with an unvaried fauna, composed throughout of the same animals, which are also for the most part identical with those of the arctic shores of Europe. It will be nearly the same along

the coast of Labrador.

416. As he approaches Newfoundland, he will see the landscape, and with it the fauna, assuming a somewhat more varied aspect. To the wide and naked or turfy plains of the boreal regions succeed forests, in which he will find

various animals which dwell only in forests. Here the tem perate fauna commences. Still the number of species is not yet very considerable; but as he advances southward, along the coasts of Nova Scotia and New England, he finds new species gradually introduced, while those of the colder regions diminish, and at length entirely disappear, some few accidental or periodical visiters excepted, who wander, during winter, as far south as the Carolinas.

417. But it is after having passed the boundaries of the United States, among the Antilles, and more especially on the southern continent, along the shores of the Orinoco and the Amazon, that our traveller will be forcibly struck with the astonishing variety of the animals which people the forests, the prairies, the rivers, and the sea-shores, most of which he will also find to be different from those of the northern continent. By this extraordinary richness of new forms, he will become sensible that he is now in the domain of the tropical fauna.

418. Let him still travel on beyond the equator towards the tropic of Capricorn, and he will again find the scene change as he enters the regions where the sun casts his rays more obliquely, and where the contrast of the seasons is more marked. The vegetation will be less luxuriant; the palms will have disappeared to make place for other trees; the animals will be less varied, and the whole picture will recall to him, in some measure, what he witnessed in the United States. He will again find himself in the temperate region, and this he will trace on, till he arrives at the extremity of the continent, the fauna and the flora becoming more and more impoverished as he approaches Cape Horn.

419. Finally, we know that there is a continent around the South Pole. Although we have as yet but very imper. fect notions respecting the animals of this inhospitable clime still, the few which have already been observed there present