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268 cal zones. It is evident then, that although climate has a certain amount of influence on the distribution of animal forms, yet geographical conditions are far more important. There is reason to believe that the direct action of climate oh animal life is far less effective than its indirect action through the limitation of the variety and quantity of vege table and insect food ; whereas geographical isolation has led to diversity of type by its influence ou development during successive ages, as pointed out by Mr Darwin (Origin of Species, 6th ed. p. 81, 83.) It follows that zoological regions, or those primary divisions of the earth characterized by distinct assemblages of animals, will, for the most part, coincide with natural geographical divisions. They do not, however, conform to the actual divisions of our geographies, because these are often political or ethno graphical, rather than physical as in the separation of Europe from Asia. In another case, the coincidence of a mountain chain (the Himalayas) and the plateau of Thibet, with the demarcation of the tropical and temperate zones, forms a zoological division across a continent almost as complete as would be effected by a considerable extent of oceau. Vertical Distribution of Animals. Besides the horizontal distribution dependent on the various causes just indicated, the range of animals is more or less determined by the altitude of the land surface above, or its depth below the sea-level. As we ascend lofty mountains, the forms of life change in a manner somewhat analogous to the changes observed in passing from a warm, to a cold country. This change is, however, far less observable in animals than in plants ; and it is so unequal in its action, and can so fre quently be traced to mere change of climate and deficiency of food, that it must rank as a phenomenon of secondary importance. Vertical distribution among animals will be found in most cases to affect species rather than generic or family groups, and to involve in each case a mass of local details which can hardly be introduced in a general sketch of the whole subject of distribution. The same remarks apply to the bathymetrical zones of marine life. Many groups are confined to tidal, or shallow, or deeper waters ; but these differences of habit are hardly " geographical," but involve details, suited rather to the special study of individual groups than to such a general outline of the dis tribution of the animal kingdom as we are here attempting to lay before our readers. Powers of Dispersal of Animals. Animals differ greatly in their powers of dispersal or migration ; and this is an important element in determining the causes of their actual distribution. Mammalia as a class are more limited in this respect than birds; because the former have no means of passing over seas and oceans, or, with few exceptions, over lofty mountains or arid deserts, all of which when of moderate width can be easily traversed by many birds. Reptiles in their adult state are almost as restricted in their powers of dispersal as mammals, but most of them being oviparous, their eggs may be floated on drift wood over seas and straits, or even, in rare cases, be carried by birds ; whereas the young of mammalia are for some time wholly dependent on their parents. Amphibia and fresh-water fishes have yet another advantage, that many of them can endure great cold, and their ova may sometimes be frozen vithout injury. Thus floating ice becomes an important agent in their dispersal, and enables us to account for the curious fact that their distribution often differs in a remark- uble manner from that of the three higher classes of verte brates. When we come to insects, we find the power of dispersal (as regards land animals) at a maximum ; for not only can they travel by almost every mode available to other groups, but their small size, low specific gravity, and (in many cases) great tenacity of life, give them altogether [ANIMAI* exceptional advantages in this respect. They are easily carried for great distances through the air by gales and storms ; and there is evidence to show that many remote islands have been thus stocked, and that many wide-spread groups owe their extensive range to this cause. Others can float uninjured for many days at sea; while their eggs or larvaj, inclosed in crevices of tree-trunks or concealed under bark, may be carried for hundreds or even thousands of miles by surface currents across extensive seas (Wallace, Geographical Distribution of Animals, vol. i. pp. 32, 209-214). The fact, then, that these small creatures have often a more extensive range, and present greater anomalies in their distribution, than larger animals, is only what we might expect; and if we keep their unusual powers of dispersal ever present to our minds, we shall be able to account for most of the anomalies they present, and thua bring them under the same general classification of the phenomena of distribution which is most serviceable in studying the history of the higher animals. But the actual power of dispersal is by no means the only factor in determining the distribution of a species or a group. It is no use to bring a creature to a new country if it cannot live and maintain itself there. Whether it can do so depends upon many causes. It must be able to adapt itself to a different climate, and generally to different physical conditions ; it must be able to live upon whatever food it may find in its new abode ; and, most important of all, it must be able to defend itself against new kinds of enemies and to live in successful com petition with allied organisms which are already in possession of the soil. Wide-spread and Local Groups. There is much reason to believe that the last-mentioned condition is the most difficult for an intruder to fulfil, and that a large propor tion of the immigrants which from any cause arrive in a new country, are unable to maintain themselves in it, not because the country itself is not well adapted to their wants, but solely because it is already occupied by other creatures somewhat better adapted to all the surrounding conditions. Hence arise the phenomena of wide-spread or dominant species, and others which are exceedingly local and often rare, that is, consisting of but a small group of individuals. The former are best adapted to the entire environment, and are generally increasing their numbers and area of distribution ; the latter are less perfectly adapted, and. probably diminishing in numbers and on the road to final extinction. The power of adaptation seems, generally speaking, to be in an inverse ratio to the power of dispersal. The larger mammalia and many birds are capable of enduring a great variety of climates, and even of maintaining themselves in many new countries in competi tion with the native inhabitants. Thus horses and cattle from the Old World have run wild and greatly multiplied in both North and South America, and are probably capable of existing in any country where there is a sufficiency of open uncultivated land. Insects, on the other hand, are often dependent on some one kind of vegetable food, are especially liable to injuries by climate, and unless very numerous would be liable to be at once exterminated by their various enemies. Barriers which Limit the Distribution of Animals. These are of many kinds, and affect the several groups in unequal degrees. The nature of the vegetation alone determines the range of a number of animals. Deserts, marshes, open plains, and especially forests, have each their peculiar inhabitants which can hardly stray far beyond their limits. This is particularly the case with the tropical forests, whose perennial foliage and almost perennial succession of flowers and fruits supply the wants of an immense number of

peculiar forms of life. These forests are, in fact, the home