is an essential unity of life over this vast area, and the recognition of North America as a separate (nearctic) realm, which some writers have attempted, seems hardly practicable.

The Neotropical or South American realm includes South America, the West Indies, the hot coast lands of Mexico, and those parts of Florida and Texas where frost does not occur. Its boundaries through Mexico are not sharply defined, and there is much overlapping of the north temperate realm along its northern limit. Its birds especially range widely through the United States in the summer migrations, and a large part of them find in the North their breeding home. Southward, the broad barrier of the two oceans keeps the South American fauna very distinct from that of Africa or Australia. The neotropical fauna is richest of all in species. The great forests of the Amazon are the dreams of the naturalists. Characteristic types among the larger animals are the snout or broad-nosed (platyrrhine) monkeys, which in many ways are very distinct from the monkeys and apes of the Old World. In many of them the tip of the tail is highly specialized and is used as a hand. The Edentates (armadillos, ant-eaters, etc.) are characteristically South American, and there are many peculiar types of birds, reptiles, fishes, and insects.

The Indo-African realm corresponds to the neotropical realm in position. It includes the greater part of Africa, merging gradually northward into the north temperate realm through the transition districts which border the Mediterranean. It includes also Arabia, India, and the neighboring islands, all that part of Asia south of the limit of frost. In monkeys, carnivora, ungulates, and reptiles this region is wonderfully rich. In variety of birds, fishes, and insects the neotropical realm exceeds it. The monkeys of this district are all of the narrow-nosed (catarrhine) type, various forms being much more nearly related to man than is the case with the peculiar monkeys of South

America. Some of these (anthropoid apes) have much in common with man, and a primitive man derived from these has been imagined by Haeckel and others. No creature of this character is yet known, but that it may have once existed is not impossible. To this region belong the elephant, the rhinoceros, and the hippopotamus, as well as the lion, tiger, leopard, giraffe, the wild asses, and horses of various species, besides a large number of ruminant animals not found in other parts of the world. It is, in fact, in its lower mammals and reptiles that its

most striking distinctive characters are found. In its fish fauna it has very much in common with South America.

The Lemurian realm comprises Madagascar alone. It is an isolated division of the IndoAfrican realm, but the presence of many species of lemur and an unspecialized or primitive type of lemur is held to justify its recognition as a distinct

realm. In most other groups of animals the fauna of Madagascar is essentially that of neighboring parts of Africa.

The Patagonian realm includes the south temperate zone of South America. It has much in common with the neotropical realm from which its fauna is mainly derived,

but the presence of frost is a barrier which vast numbers of species can not cross. Beyond the Patagonian realm lics the Antarctic continent. The scanty fauna of this region is little known, and it probably differs from the Patagonian fauna chiefly in the absence of all but the ice-riding species.

The Australian realm comprises Australia and the neighboring islands. It is more isolated than any of the others, having been protected by the sea from the invasions of the characteristic animals of the Indo-African and temperate realms. It shows a singular persistence of low or primitive types of vertebrate life, as though in the process of evolution the region had been left a whole geological age behind the others. It is certain that if the closely competing fauna of Africa and India could have been able to invade Australia, the dominant mammals and birds of that region would not have been left as they are now—marsupials and parrots.

It is only when barriers have shut out competition that simple or unspecialized types abound. The larger the land area and the more varied its surface, the greater is the stress of competition and the more specialized are its characteristic forms. As part of this specialization is in the direction of hardiness and power to persist, the species from the large areas, as a whole, are least easy of extermination. The rapid multiplication of rabbits and foxes in Australia, when introduced by the hand of man, shows what might have taken place in this country had not impassable barriers of ocean shut them out.

160. Subordinate realms or provinces.—Each of these great realms may be indefinitely subdivided into provinces and sections, for there is no end to the possibility of analysis. No school district has exactly the same animals or plants as any other, as finally in ultimate analysis we find that no two animals or plants are exactly alike. Shut off one pair of animals from the others of its species, and its descendants will differ from the parent stock. This differ

ence increases with time and with distance so long as the separation is maintained. Hence new species and new fauna or aggregations of species are produced wherever free diffusion is checked by any kind of barrier.

161. Faunal areas of the sea.-In like manner, we may divide the oceans into faunal areas or zones, according to the distribution of its animals. For this purpose the fishes probably furnish the best indications, although results very similar are obtained when we consider the mollusks or the crustacea. The fresh-water fishes are not considered here, as in regard to their faunal areas they agree with the land animals of the same regions. Perhaps the most important basis for primary divisions is found in the separation from the localized shore-fishes of the cosmopolitan pelagic species, and the scarcely less widely distributed bassalian species or fishes of the deep sea.

The pelagic fishes are those which inhabit the open sea, swimming near the surface, and often in great schools. Such forms are mainly confined to the warmer waters. They are for the most part predatory fishes, strong swimmers, and many of the species are found in all warm seas. Most species have special homing waters, to which they repair in the spawning season. Often there will be special regions to which they never resort, either for breeding or for food. At other times a certain species will appear in numbers in regions where it has hitherto been unknown. For example, the frigate-mackerel (Auxis thazard), homing in the East Indies and the Mediterranean, appeared in great numbers in 1880 off the coast of New England. Typical pelagic fishes are the mackerel, tunny, dolphin, flyingfish, opah, and some species of shark. This group shades off by degrees into the ordinary shore-fish, some being partly pelagic, venturing out for short distances, and some are pelagic for part of the year only. To the free-swimming forms of classes of animals lower than fishes, found in the open ocean, the name Plankton is applied.

The bassalian fauna, or deep-sea fauna, is composed of species inhabiting great depths (2,500 feet to 25,000 feet) in the sea. At a short distance below the surface the change in temperature from day to night is no longer felt. At a still lower depth there is no difference between winter and summer, and still lower none between day and night. The bassalian fishes in

habit a region of great cold and inky darkness. Their bodies are subjected to great pressure, and the conditions of life are practically unvarying. There is therefore among them no migration, no seasonal change, no spawning season fixed by outside conditions, and no need of adaptation to varying environment. As a result, all are uniform indigo-black in color, and all show more or less degeneration in those characters associated with ordinary environment. Their bodies are elongate, from the lack of specialization in the vertebræ. The flesh, being held in place by the great

ocean.

pressure of the water, is soft and fragile. The organs of touch are often highly developed. The eye is either excessively large, as if to catch the slightest ray of light, or else it is undeveloped, as if the fish had abandoned the effort to see. In many cases luminous spots or lanterns are developed by which the fish may see to guide his way in the