Page:Encyclopædia Britannica, Ninth Edition, v. 15.djvu/395

Rh DISTRIBUTION.] MAMMALIA 373 them and all other mammals that, in the absence of any knowledge of intermediate or transitional forms, they can be perfectly separated, and form as well denned an order as any in the class. Lastly, we have the important and well-characterized group, called Primates, including all the Monkeys and Man, and the question is not yet solved as to how and through what forms it is linked on to the other groups. It is commonly assumed that the Lemurs are nothing more than inferior Primates, but the interval between them in the actual fauna of the world is very great, and our knowledge of numerous extinct species recently discovered in America, said to be intermediate in characters, is not yet sufficiently perfect to enable us to form a definite opinion upon the subject. The distinctive characters of the generally recognized orders of mammals, with an account of their subdivisions and the principal forms contained in each, will be given further on. GEOGRAPHICAL DISTRIBUTION OF MAMMALIA. The existing species of few classes of the animal kingdom are better known than those of the Mammalia, and, owing to the comparatively limitad methods of locomotion or transport which most of them possess, the area of distribu tion of each species is more definite and restricted than in some other classes, In the articles BIRDS and DISTRIBU TION the various regions into which naturalists have divided the earth s surface, according to the prevailing characters of its animal inhabitants, have been described, and in the latter the main facts connected with the distribution of mammals have been treated of. In the account of each group contained in the present article the particular circumstances relating to its geographical range will be mentioned. There is little therefore needed here, except a brief summary of the most important facts relat ing to this interesting subject. As regards their distribution over the surface of the earth, mammals may be divided into three groups accord ing to their principal methods of locomotion (1) aerial, (2) aquatic, and (3) terrestrial. .rial 1. Aerial Mammals. This group only comprises the urn- animals composing the single order Chiroptera, which
 * ils&amp;gt; differ from all other mammals in the fact that their prin

cipal means of transport from place to place is by aerial flight, as in the majority of birds and insects. Broad expanses of water, which form natural barriers to the spread of terrestrial mammals, are therefore no obstacles to their distribution ; accordingly we find the general rule that mammals are not inhabitants of oceanic islands modi fied in their case. But even in this group, notwithstanding their exceptional powers of locomotion, different species, genera, and families inhabit very definite areas. Each zoological region of the earth has its characteristic Bats ; and those of the New World and of the Old World are, with very few exceptions, quite distinct. This subject will be more fully treated of under the order Chiroptera. }uatic 2. Aquatic Mammals. Many mammals grouped for the ~ present purpose as terrestrial pass a great portion of their 1 s&amp;lt; lives in brooks, lakes, or rivers, and, being dependent upon such waters for obtaining their subsistence, are necessarily confined to their vicinity ; but the truly aquatic mammals, or those living constantly in the water, and unable to move their quarters from place to place by land, are the orders Cetacea and Sirenia, with which may also be grouped the Seals, forming the Pinniped division of the order Carnivora. For the marine Cetacea, animals mostly of large size and endowed with powers of rapid locomotion, there are obviously no barriers to universal distribution over the surface of the earth covered by sea, except such as are interposed by uncongenial temperature or absence of suit able food. Nevertheless it was thought some years ago that the fact of a Whale or a Dolphin occurring in a sea distant from that in which it had usually been found was sufficient justification for considering it as a distinct species and im posing a new name upon it. There are now, however, so many cases known in which Cetaceans from the northern and southern seas, from the Atlantic and the Pacific Oceany, present absolutely no distinguishing external or anatomical characters upon which specific determination can be based that the opposite view is gaining ground ; and, as some species are undoubtedly very widely distributed, almost cosmopolitan in fact, there seems little reason why many others should not be included in the same category. The evidence is satisfactory enough in those cases in which the intermediate regions are inhabited by the same forms, the cases of &quot;continuous areas&quot; of distribution. In those in which the arsas of distribution are apparently discontinuous, there may be more room for doubt ; but it must not be forgotten that the negative evidence is here of much less value than in the case of land animals, as the existence of Cetaceans in any particular part of the ocean is most &quot;easily overlooked. The great Sperm Whale (Physeter macro- cephalus) is known to be almost cosmopolitan, inhabiting or passing through all the tropical and temperate seas, although not found near either pole. At least three of the well-known species of Rorqual (Bal&noptera) of the British coasts are represented in the North Pacific, on the South American shores, and near New Zealand by species so closely allied that it is difficult to point out any valid dis tinctive characters, though it may perhaps be desirable to wait for a more complete examination of a large series of individuals before absolutely pronouncing them to be specifically identical. There is nothing yet known by which we can separate the / Humpback Whales &quot; (Mega- ptera) of Greenland, the Cape of Good Hope, and Japan. The same may be said of the common Dolphin of the European seas (Delphimis delphis) and the so-called D. bairdii of the North Pacific and D. forsteri of the Australian seas. The Pilot Whale (Globicephalus melas) and the Pseudorca of the North Atlantic and of New Zealand are also precisely alike, as far as present know ledge enables us to judge. Many other similar cases might be given. Captain Maury collected much valuable evidence about the distribution of the larger Cetacea, and, finding Right Whales (a/sena) common in both northern and southern temperate seas, and absent in the intermediate region, laid down the axiom that &quot; the torrid zone is to the Right Whale as a sea of fire, through which he cannot pass.&quot; Hence all cetologists have assumed that the Right Whale of the North Atlantic (B. liscayensis), that of the South Seas (B. australis), and that of the North Pacific (B. japonica) are necessarily distinct species. The anato mical structure and external appearance of all are, how ever, as far as yet known, marvellously alike, and, unless some distinguishing characters can be pointed out, it seems scarcely justifiable to separate them upon geographical position alone ; as, although the tropical seas may be usually avoided by them, it scarcely seems impossible, or even improbable, that some individuals of animals of such size and rapid powers of swimming may not have at some time traversed so small a space of ocean as that which divides the present habitual localities of these supposed distinct species. If identity or diversity of struc tural characters is not to be allowed as a test of species in these cases, as it is usually admitted to be in others, the study of their geographical distribution becomes an impossibility. Although many species are thus apparently of such wide distribution, others are certainly restricted ; thus the