form with the climatic zones, though not always with the parallels of the geographer."

"In fact, so generally is temperature recognized by the leading writers on the distribution of marine life that it seems superfluous to reiterate or emphasize this principle. That the zones of life should be perhaps a little less obvious over the land areas--in consequence of the diversity of contour resulting from differences of elevation, and the interruptions and exceptional conditions due to mountain chains and high plateaus-than over the oceanic expanses, is naturally to be expected. That there is, however, a similar correspondence between climatic belts and the zones of life seems to me abundantly evident. As has been already shown, the broader or primary zones are, first, an Arctic or North Circumpolar Zone, embracing the arctic, subarctic, and colder temperate latitudes of the northern hemisphere, throughout the whole of which area there is a marked homogeneity of mammalian life, as well as of animal and vegetable life in general; secondly, that below this there is a broad belt of life, which, in its general facies, is distinctive of the temperate and warm-temperate latitudes, and that these two zones of life are far more closely related inter se than with the life of the intertropical regions, with which regions they may be collectively contrasted, and together receive the appropriate name of Arctogaa;' thirdly, it has been shown, so far as the northern hemisphere is concerned, that the life of the tropical and temperate regions of the same continent is more widely different than is the life of corresponding portions of the temperate and colder parts of the (so-called) Old World and the New; fourthly, that the life of Tropical America has very little in common with that of the tropical portions of Asia and Africa; fifthly, that the life of the South Temperate Zone presents a facies distinct from that of the tropics, and has still less in common with that of the North Temperate Zone; Sixthly, that Australasia is so highly differentiated as to form a distinct primary region, having little in common with other lands, even with those of contiguous regions, or those having a similar geographical position; seventhly, that Madagascar and its contiguous islands, while to some extent African in affinity, form also a highly specialized region; lastly, that the antarctic and cold south

* Allen, op. cit., PP. 373-375.

temperate oceanic regions are recognizable as a primary region, characterized by a peculiar general facies of life, that more strongly recalls that of the corresponding portions of the northern hemisphere than of any other portion of the earth. It has been further shown that the Australian Realm is divisible into temperate and tropical portions, and also that the land surface is separable into zones of even still narrower limits, corresponding in a general way with those recognized by Dana for marine life.

"The almost total absence of identical genera, or even of families, excepting such as are essentially cosmopolitan, in the American and Old World tropics, as well as the distinctness of the Lemurian Realm, and the almost total isolation of the Australian Realm, evidently require for their explanation other causes than merely the existing climates. The geological history of these land-areas and their faunæ must be of course considered in order to understand their present relationships. As the northern hemisphere at present most clearly shows, nearly continuous land surface and similarity of climatic conditions implies identity of fauna, while isolation, especially when joined with diverse climatic conditions, implies diversity of life, and a differentiation proportionate to the degree of isolation, and the length of time such isolation has existed; in other words, that the present want of affinity between the life of the Lemurian and Australian Realms and that of the rest of the world is due rather to their long geographical isolation than to present climatic conditions, and that we here find, for reasons perhaps not wholly apparent, the remnants of a somewhat primitive or early fauna that was formerly shared more largely by other areas than at present— that these regions became isolated before the development of many of the higher and now prevalent types of the larger and more diversified land-areas, and that here differentiation has proceeded less rapidly and along fewer and narrower lines than elsewhere; furthermore, that the present highly diversified fauna of the chief tropical areas, in comparison with the fauna of the north-circumpolar lands, is due in part to the southward migration, near the close of the Tertiary period, of forms adapted to a high temperature, and in part to the high rate of differentiation favored by tropical conditions of climate. Hence, given: 1. Arctic and cold-temperate conditions of climate, and we have a fauna only slightly or moderately diversified; 2. A moderate increase of temperature, giving warm-temperate conditions of climate, and we have the addition of

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.

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 answer. Naturalists 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 portion 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