ZOOGEOGRAPHY, the science of map ping out the surface of the earth with reference to its fauna; the study of the distribution of animal life. It has always been obvious to trirs elers, and even the most superficial students of nature, that the various regions of the earth's surface were characterized, among other efiffer ences, by local peculiarities in animals and plants. It is a commonplace of knowledge that the animal life of the tropics is different f rola that of the arctic regions; that the birds and mammals and small creatures of Africa differ almost altogether from those a South Amer ica or Australia. Closer examination shows that such differences exist in a greater or less degree between lesser regions, as the east and west sides of a groups of islands separated by a sea-space, and so forth. On the other hand, there may be found striking re semblances in the faunae of certain regions, or a sameness over an extensive area, as Europe and Asia.
The importance and significance of these facts impressed themselves upon scientific men only within recent times. As long as it was held that each species must have been created, as a general rule within the geographical area which it now occupies, the most curious facts of distribution could be regarded only with 'sterile wonder.• But when the idea came to be entertained that allied species have had a common origin, it was obviously implied that they or their ancestors must have had a com mon birthplace, and consequently, when we find members of a from their nearest kindred, we feel to inquire how this came about. Thus, we have to explain how the tapirs are confined to the Malayan region and South America; the camels to the deserts of Asia and the Andes; marsupials to the Australian region and America; how the birds, mammals and reptiles of North America resemble those of Europe more than those of South America and so on.
Means of Dispersal.—Accepting as a start ing point the proposition that the various forms of life originated in some parent-stock or stocks at a particular place or places, the pres ent distribution of their descendants as we know them, must depend mainly upon the pow ers of dispersion which each possessed, and the character of the physical influences and ob structions which acted as controlling factors or barriers in guiding their dispersal along cer tain lines and altogether prevented it elsewhere.
Theoretically a new stock would spread equally in all directions from its point of origin; actu ally a very irregular and complicated kind of distribution has ensued in most or all cases. Hence an inquiry as to the means and limita tions of dispersion possessed by animals and plants is of first importance.
It is scarcely necessary to draw attention to the facilities for diffusion possessed .by ani mals endowed with great locomotive powers, and especially among land animals, by those having the power of flight (q.v.); but it is im portant to note that some animals, which in the adult state have only feeble powers of locomo tion, are better endowed in this respect when young. Such, for example, is the case with echinoderms, marine mollusks and similar lowly aquatic forms, all of which develop from free swimming and often far-drifting larva. (See line-Saw Lire). But accidental modes of dis persal must also be taken into account. The carrying power of winds is known to be suffi cient to bear along in the air fine dust across seas many hundreds of miles in width, and we have in that agency alone an adequate means of accounting for the dispersion of all plants propagated by minute spores. For that reason the distribution of most cryptogamic plants hardly enters into the problem, since these mosses, fungi, seaw eeds and the like are almost universal. What' part winds may have played the seeds of flowering plants is more doubtful; but observations show• that even for such seeds, especially when provided with some kind of feathery appendage, winds may occasionally serve as a means of transport for very long distances. Sec Pewter Gem