possession, and their descendants are thus native to the islands. But, isolated from the great mass of their species and bred under new surroundings, these island birds come to differ from their parents, and still more from the great mass of the land species of which their ancestors were members. Separated from these, their individuality would manifest itself. They would assume with new environment new friends, new foes, new conditions. They would develop qualities peculiar to themselves-qualities intensified by isolation. Local peculiarities disappear with wide association, and are intensified when individuals of similar peculiarities are kept together. Should later migrations of the original species come to the islands, the individuals surviving would in time form new species, or, more likely, mixing with the mass of those already arrived, their special characters would be lost in those of the majority. The Galapagos, first studied by Darwin, serve to us only as an illustration. The same problems come up, in one guise or another, in all questions of geographical distribution, whether on continent or island. The relation of the fauna of one region to that of another depends on the ease with which barriers may be crossed. Distinctness is in direct proportion to isolation. What is true in this regard of the fauna of any region as a whole, is likewise true of any of its individual species. The degree of resemblance among individuals is in direct proportion to the freedom of their movements, and variations within what we call specific limits is again proportionate to the barriers which prevent equal and perfect diffusion. The laws governing the distribution of animals are reducible. to three very simple propositions. Every species of animal is found in every part of the earth having conditions suitable for its maintenance unless: (a) Its individuals have been unable to reach this region, through barriers of some sort; or, (b) Having reached it, the species is unable to maintain itself, through lack of capacity for adaptation, through severity of competition with other forms, or through destructive condition of environment; or, (c) Having entered and maintained itself, it has become so altered in the process of adaptation as to become a species distinct from the original type. As examples of the first class we may take the absence of kingbirds or meadow larks or coyotes in Europe, the absence of the lion and tiger in South America, the absence of the civet cat in New York, and that of the bobolink or the Chinese flying fox in California. In each of these cases there is no evident reason why the species in question should not maintain itself if once introduced. The fact that it does not exist is, in general, an evidence that it has never passed the barriers which separate the region in question from its original home. Local illustrations of the same kind may be found in mountainous regions. In the Yosemite Valley in California, for example, the trout ascend the Merced River to the base of the Vernal fall. They cannot rise above this and so the streams and lakes above this fall are destitute of fish. Examples of the second class are seen in animals that man has introduced from one country to another. The nightingale, the starling, and the skylark of Europe have been repeatedly set free in the United States. But none of these colonies has long endured; perhaps from lack of adaptation to the climate, perhaps from severity of competition with other birds, most likely because the few individuals become so widely scattered that they do not find one another at mating time. In other cases the introduced species has been better fitted for the conditions of life than the native forms themselves, and so has gradually crowded out the latter. Both these cases are illustrated among the rats. The black rat (Mus rattus), first introduced into America from Europe about 1544, tended to crowd out the native wild rats (Sigmodon), while the brown rat (Mus decumanus), brought in still later, about 1775, in turn practically exterminated the black rat, its fitness for the conditions of life here being greater than that of the other European species. Of the third class, or species altered in a new environment, examples are numerous, but in most cases the causes involved can only be inferred from their effects. One class of illustrations may be taken from island faunas. An island is set off from the mainland by barriers which species of land animals can very rarely cross. On an island a few waifs may maintain themselves, increasing in numbers so as to occupy the territory, but in so doing only those kinds will survive that can fit themselves to the new conditions. Through this process new species will be formed, like the parent species in general structure, but having gained new traits adjusted to the new environment. To processes of this kind, on a larger or smaller scale, the variety in the animal life of the globe must be largely due. Isolation and adaptation through selection probably give the clew to the formation of a very large proportion of the "new species" in any group. It will be thus seen that geographical distribution is primarily dependent on barriers or checks to the movement of animals. The obstacles met in the spread of animals determine the limits of the species. Each species broadens its range as far as it can. It attempts, unwittingly, of course, through natural processes of increase, to overcome the obstacles of ocean and river, of mountain or plain, of woodland or prairie or desert, of cold or heat, of lack of food, or abundance of enemies-whatever the barriers may be. Were it not for these barriers, each type or species would become cosmopolitan or universal. Man is preeminently a barrier-crossing animal; hence, in different races or species, man is found in all regions where human life is possible. The different races of men, however, find checks and barriers entirely similar in nature to those experienced by the lower animals, and the race peculiarities are wholly similar to characters acquired by new species under adaptation to changed conditions. The degree of hindrance offered by any barrier differs with the nature of the species trying to surmount it. That which constitutes an impassable obstacle to one form may be a great aid to another. The river which blocks the monkey or the cat is the highway of the fish or the turtle. The waterfall which limits the ascent of the fish is the chosen home of the ouzel. The mountain barrier which the bobolink or the prairie dog does not cross may be the center of distribution of the little chief hare or the Arctic bluebird. The term fauna is applied to the animals of any region considered collectively. Thus the fauna of Illinois comprises the entire list of animals found naturally in that State. It includes the aboriginal man, the black bear, the fox, and all its animal life down to the Amaba and the microbe of malaria. The relation of the fauna of one region to that of another depends on the ease with which barriers may be crossed. Thus the fauna of Illinois differs little from that of Indiana or Iowa, because the State contains no barriers that animals may not readily pass. On the other hand, the fauna of California or Colorado differs materially from that of the adjoining regions, because the mountainous country is full of barriers which FIG. 186.-Two Ocean Pass, the summit of the Continental Divide between the Yellowstone and Snake rivers; obstruct the diffusion of life. Distinctness is in direct proportion to isolation. What is true in this regard of the fauna of any region is likewise true of its individual species. The degree of resemblance among individuals is in strict proportion to the freedom of their movements. Variation within the limits of a species is again proportionate to the barriers which prevent equal and free diffusion. The various divisions or realms into which the land surface of the earth may be divided, on the basis of the character of the animal life, have their boundary in the obstacles offered to the spread of the average animal. In spite of great inequalities in this regard, we may yet roughly divide the land of the globe into seven principal realms or areas of distribution, each limited by barriers, of which the chief are the presence of the sea and the occurrence of frost. There are the Arctic, North Temperate, South American, Indo-African, Patagonian, Lemurian, and Australian realms. Of these the Australian realm alone is sharply defined. Most of the others are surrounded by a broad fringe of debatable ground that forms a transition to some other zone. The Arctic realm includes all the land area north of the isotherm 32°. Its southern boundary corresponds closely with the northern limit of trees. The fauna of this region is very homogeneous. It is not rich in species, most of the common types of life of warmer regions being excluded by the cold. Among the large animals are the polar bear, the walrus, and certain species of "ice-riding" seals. There are a few species of fishes, mostly trout and sculpins, and a few insects; some of these, as the mosquito, are excessively numerous in individuals. Reptiles are absent from this region and many of its birds. migrate southward in the winter, finding in the Arctic their breeding homes only. When we consider the distribution of insects and other small animals of wide diffusion we must add to the Arctic realm all high mountains of other realms whose summits rise above the timber line. The characteristic large. animals of the Arctic, as the polar bear or the musk-ox or the reindeer, are not found on the mountain tops because barriers shut them off. But the Alpine flora, even under the equator, may be characteristically arctic, and with the flowers of the north may be found the northern insects on whose presence the flowers depend for their fertilization and which in turn depend on these for their food. So far as climate is concerned, high altitude is equivalent to high latitude. On certain mountains the different zones of altitude and the corresponding |