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391. If it be ths shown that the transformations which take place in the embryo are of the same nature, and of the same importance, as those which occur afterwards, the circumstance that some precede and others succeed birth cannot mark any radical distinction between them. Both are processes of the life of the individual. Now, as life does not commence at birth, but goes still farther back, it is quite clear that the modifications which supervene during the former period are essentially the same as, and continuous with, the later ones; and hence, that metamorphoses, far from being exceptional in the case of Insects, are one of the general features of the Animal Kingdom.

392. We are, therefore, perfectly entitled to say that all animals, without exception, undergo metamorphoses. Were it not so, we should be at a loss to conceive why animals of the same division present such wide differences; and that there should be, as in the class of Reptiles, some families that undergo important metamorphoses, (the frogs, for example,) and others in which nothing of the kind is observed after birth, (the Lizards and Tortoises.)

393. It is only by connecting the two kinds of transformations, namely, those which take place before, and those after birth, that we are furnished with the means of ascertaining the relative perfection of an animal; in other words, these transformations become, under such circumstances, a natura. key to the gradation of types. At the same time, they will force upon us the conviction that there is an immutable principle presiding over all these changes, and regulating them in a peculiar manner in each animal.

394. These considerations are exceedingly important, not only from their bearing upon classification, but not less so from the application which may be made of them to the study of fossils. If we examine attentively the fishes that have been found in the different strata of the earth, we remark that

those of the most ancient deposits have, in general, preserved only the apophyses of their vertebræ, whilst the vertebræ then selves are wanting. Were the Sturgeons of the American rivers to become petrified, they would be found in a similar state of preservation. As the apophyses are the only bony portions of the vertebral column, they alone would be preserved. Indeed, fossil Sturgeons are known, which are in precisely this condition.

395. From the fact above stated, we may conclude that the oldest fossil fishes did not pass through all the metamorphoses which our osseous fishes undergo; and, consequently, that they were inferior to analogous species of the present epoch which have bony vertebræ. Similar considerations apply to the fossil crustacea and to the fossil Echinoderms, when compared with living ones, and will, probably, be true of all classes of the Animal Kingdom, when fully studied as to their geological succession.

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CHAPTER THIRTEENTH.

GEOGRAPHICAL DISTRIBUTION OF ANIMALS.

SECTION I.

GENERAL LAWS OF DISTRIBUTION.

396 No animal, excepting man, inhabits every part of the surface of the earth. Each great geographical or climatal region is occupied by some species not found elsewhere; and each animal dwells within certain limits, beyond which it does not range while left to its natural freedom, and within which it always inclines to return, when removed by accident or design. Man alone is a cosmopolite. His domain is the whole earth. For him, and with a view to him, it was created. His right to it is based upon his organization and his relation to Nature, and is maintained by his intelligence and the perfectibility of his social condition.

397. A group of animals which inhabits any particular region, embracing all the species, both aquatic and terrestrial, is called its FAUNA; in the same manner as the plants of a country are called its Flora. To be entitled to this name, it is not necessary that none of the animals composing the group should be found in any other region; it is sufficient that there should be peculiarities in the distribution of the families, genera, and species, and in the preponderance of certain types over others, sufficiently prominent to impress upon a region well-marked features. Thus, for example, in the islands of the Pacific are found terrestrial animals, alto

gether peculiar, and not found on the nearest continents. There are numerous animals in New Holland differing from any found on the continent of Asia, or, indeed, on any other part of the earth. If, however, some species inhabiting both shores of a sea which separates two terrestrial regions are found to be alike, we are not to conclude that those regions have the same Fauna, any more than that the Flora of Lapland and England are alike, because some of the sea-weeds found on both their shores are the same.

398. There is an evident relation between the fauna of any locality and its temperature, although, as we shall hereafter see, similar climates are not always inhabited by similar animals, (401, 402.) Hence the faunas of the two hemispheres have been distributed into three principal divisions, namely, the arctic, the temperate, and the tropical faunas: in the same manner as we have arctic, temperate, and tropical floras. Hence, also, animals dwelling at high elevations upon mountains, where the temperature is much reduced, resemble the animals of colder latitudes, rather than those of the surrounding plains.

399. In some respects, the peculiarities of the fauna of a region depend upon its flora, at least so far as land animals are concerned; for herbivorous animals will exist only where there is an adequate supply of vegetable food. But taking the terrestrial and aquatic animals together, the limitation of a fauna is less intimately dependent on climate than that of a flora. Plants, in truth, are for the most part terrestrial, (marine plants being relatively very few,) while animals are chiefly aquatic. The ocean is the true home of the Animal Kingdom; and while plants, with the exception of the lichens and mosses, become dwarfed, or perish under the influence of severe cold, the sea teems with animals of all classes, far beyond the extreme limit of flowering plants.

400. The influence of climate, in the colder regions, acts merely to induce a greater uniformity in the species of animals. Thus the same animals inhabit the northern polar regions of the three continents. The polar bear is the same in Europe, Asia, and America, and so are also a great many birds. In the temperate regions, on the contrary, the species differ on each of the continents, but they still preserve the same general features. The types are the same, but they are represented by quite different species. In consequence of these general resemblances, the first colonists of New England erroneously applied the names of European species to American animals. Similar differences are observed in distant regions of the same continent, within the same parallels of latitude. The animals of Oregon and of California are not the same as those of New England. The difference, in certain respects, is even greater than between the animals of New England and Europe. In like manner, the animals of temperate Asia differ more from those of Europe than they do from those of America.

401. Under the torrid zone, the Animal Kingdom, as well as the Vegetable, attains its highest development. The ani mals of the tropics are not only different from those of the temperate zone, but, moreover, they present the greatest variety among themselves. The most gracefully proportioned forms are found by the side of the most grotesque, decked with every combination of brilliant coloring. At the same time, the contrast between the animals of different continents is more marked; and, in many respects, the animals of the different tropical faunas differ not less from each other than from those of the temperate or frozen zones. Thus, the fauna of Brazil varies as much from that of Central Africa as from that of the United States.

402. This diversity upon different continents cannot de. pend simply on any influence of the climate of the tropics

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