many new types of life; 3. A high increase of temperature, giving tropical conditions of climate, and we have a rapid multiplication of new forms and a maximum of differentiation. Again, given: 1. A long-continued continuity of land surface, and we have an essential identity of fauna; 2. A divergence and partial isolation of landareas, and we find a moderate but decided differentiation of faunæ ; 3. A total isolation of land-areas, and we have a thorough and radical differentiation of fauna, proportioned to the length of time the isolation has continued. Hence, the present diversity of life is correlated with two fundamental conditions: 1. Continuity or isolation, past as well as present, of land surface; and, 2. Climatic conditions, as determined mainly by temperature."

Without further comment, we will proceed to the consideration (1) of the several regions concerning which there is proximate agreement, and (2) next to those in dispute.

THE NORTH AMERICAN TEMPERATE REALM OR NEARCTIC REGION.

It is with its widest limits that this territory has been admitted by Mr. Wallace, while by Mr. Allen it is deprived of the Arctic region, which has been associated with the isothermal portion of the Eurasia to constitute together an Arctic realm. This will be the subject of consideration hereafter. Issue has also been joined as to the southern limits of the realm and as to the pertinence or nonpertinence to it of the Sonoran and Lower Californian "regions" of Cope, but this is a question of detail which need not detain us at the present time.

THE EUROPÆO-ASIATIC OR PALÆARCTIC REALM.

The only serious point at issue between Messrs. Wallace and Allen affecting this realm is whether the Arctic portion is, or is not, an integer, Mr. Wallace including it and Mr. Allen excluding and uniting it with the American Arctic, and considering the two as the components of a "realm," as will be hereafter seen.

It will be now in order to inquire into the tenability of the other realms whose adoption has been urged by Mr. Allen. These are

the Arctic, the Indo-African, the Lemurian, the South American Temperate, and the Antarctic.

swer.

THE ARCTIC REALM.

Mr. Allen gives the following reasons for retention of this realm: "Whether or not an Arctic Region should be recognized as a division of the first rank is a question not easy to satisfactorily anNaturalists who have made the distribution of animal life in the boreal regions a subject of special study very generally agree in the recognition of a hyperboreal or circumpolar fauna, extending in some cases far southward over the Temperate Zone. The Arctic portion of this hyperborean region has been frequently set off as a secondary division, or sub-region, and generally recognized as possessing many features not shared by the contiguous region to the southward. For the present I prefer to still retain it as a division of the first rank. It is characterized mainly by the paucity of its life, as compared with every region except the Antarctic, and by what it has not rather than by the possession of peculiar species or groups. It wholly lacks both Amphibian and Reptilian life, is almost exclusively the summer home of many birds, and forms the habitat of the Esquimaux, the Arctic Fox, the Polar Bear, the Musk Ox, the Polar Hare, the Lemmings, the Walruses, the Narwhal, and the White Whale, which are confined within it. It has no Chiroptera nor Insectivora, two or three species of Shrews, however, barely reaching its southern border. It shares with the cold-temperate belt the presence of the Moose and the Reindeer, several Pinnipeds, a number of boreal species of Glires, several fur-bearing Carnivora, and a considerable number of birds. Its southern boundary may be considered as coinciding very nearly with the northern limit of arboreal vegetation, and hence approximately with the isotherm of 32° F. Its more characteristic terrestrial forms range throughout its extent, none being restricted to either the North American or Europao-Asiatic continent. Hence it is indivisible into regions of the second and third grades (regions and provinces,) and may be considered as embracing a single hyperborean assemblage of life."

It cannot be overlooked that the reasons thus urged are very unsatisfactory, and result in part from the confusion of inland and marine faunæ under the same category. The seals, walruses, and

cetaceans are not terrestrial mammals, but marine, and their distribution is governed by the same laws which affect marine animals generally. The very few peculiar species, except the musk ox, are but little modified relations of forms common to the adjoining realms, and the absence of most forms is evidently dependent on the cold climate, and furnishes no more reason for assigning a primary rank to the territory so characterized than it would to the mountain peaks and deserts so frequently isolated in the midst of the adjoining regions, and which are equally distinguished by the paucity of their animal life. The fact that it cannot be distinctly relegated to either the North American or Eurasian realms, but is neutral territory, is scarcely sufficient to warrant its entire isolation from both.

The next disputed question involves the union or distinction of the Indian and African territories. The question is thus discussed by Mr. Allen.

INDO-AFRICAN REALM.

According to Mr. Allen, "The Indo-African Realm consists mainly of Intertropical Africa and Intertropical Asia, to which it seems proper to add Extratropical South Africa. The small por

tion of Africa south of the Southern Tropic lies wholly within the warm-temperate zone. Its small extent and broad connection with Tropical Africa render its separation as a distinct realm (as I at one time rather hastily considered it) almost inadmissible, since it is especially open to the influence of the great intertropical African fauna, as is shown by the extension of many tropical forms down to within a few degrees of its southern extremity. The area really possessing a temperate climate is restricted to its extreme southern border, where alone appear the few generic and family types that do not have a very general range over the tropical portions of the Continent. This area is many times smaller than the temperate portion of South America, but, though so small, has quite a number of peculiar genera, which impart to it quite distinctive features. It yet seems better to regard it as an appendage of the great IndoAfrican Realm rather than as a distinct primary region. Madagascar, with the Mascarene Islands, on the other hand, while perhaps possessing a closer affinity with Africa than with any other conti

nental region, has yet a fauna made up so largely of peculiar types that it seems more in accordance with the facts of distribution to regard it as a separate primary region.

"The Indo-African Realm, as thus restricted, forms a highly natural division. Although its two principal areas are quite widely separated, being in fact geographically almost wholly disassociated, they possess a wonderful degree of similarity. Of the fifty commonly recognized families of mammalia occurring within its limits, three-fifths are distributed throughout almost its whole extent. Of the remainder, one-half are confined to Africa, and one is African and American, leaving only nine in India that are unrepresented in Africa; three only of these latter are, however, peculiar to the Indian Region; all extend beyond it to the northward, five of them even occurring over the greater part of the northern hemisphere. Thus the African region is the more specialized division, only a small portion of the tropical element in the Indian Region, through which it is differentiated from the great Europæo-Asiatic Temperate Region, being unrepresented in the African, while the African. has three times as many peculiar families as the Indian.”

I am quite unable to appreciate the force of this exposition as an argument in favor of the union of the two regions; it appears to me that it is, indeed, one that tells for the contrary side. Let it be recalled that the ten families* peculiar to the African region are very distinct, and that almost all of the eighteen families "common to both regions" can be added to the twelve "of wide extralimital range," if we take into consideration their distribution in even newer Tertiary or sometimes Quaternary times. Further, the genera even were, for the most part, of wide distribution formerly, and there is strong reason to believe that the thirty forms "common to both regions" were invaders of Africa in the later Tertiary, and that among those now "peculiar to the African region" we have the remnants of older fauna. If we revert to the fishes we find some striking facts. These can be resolved under two categories. On the one hand a number of forms are peculiar to Africa, or shared in common with South America; on the other are certain genera

* There are really more.

shared in common with Asia, or very closely related to Asiatic forms, and well fitted for extension of their range by tenacity of life or adaptation for limited ærial respiration. The evidence here again leads to the conclusion that the peculiar types are derived from very ancient tenants of the territory, while those common to Asia are of recent introduction. We must of course take cognizance of these contrary indications in our appreciation of the relations of the respective regions, and not allow ourselves to be unduly influenced by the predominance of the recent invaders. Africa is a decidedly distinct region so far as its aboriginal population is concerned. Further, its relations, as indicated by its primitive and more characteristic types, are with South America rather than with India, as I shall hereafter show.

THE LEMURIAN OR MALAGASY REALM.

Whether the Malagasy region or Lemurian realm of Allen is independent or an appanage of the African, is the question naturally next in order.

According to Mr. Allen, "As was long since claimed by Dr. Sclater,* Madagascar is faunally so distinct from every other ontological division of the globe as to be entitled to the rank of a primary zoogeographical region. With it, as is generally admitted, should be associated the Mascarene Islands. The very few mammals indigenous to these islands are decidedly Madagascarine in their affinities, as are the birds and other land animals. While the Lemurian fauna shows decided African affinities, it is second only to the Australian in its degree of specialization. It departs most strikingly from all other regions in what it lacks, through the absence of all Carnivores save one peculiar family (Cryptoproctide), represented by a single species, and four peculiar genera of the family Viverride; of all Ruminants and Proboscidians; all Pachyderms, except a single African genus of Suida; and all Rodents, except a few species of Murida. The Insectivores are almost wholly represented by one or two species of Crocidura, and a family, embracing several genera, not found elsewhere, save a single genus in

*Quart. Journ. Sci., vol. i, April, 1864, pp. 213–219 (Allen).*