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GEOGKAPHY OF MAMMALS
(plate L, p. 16)
It has long been evident to naturalists that the ordinary political divisions of the earth's surface do not correspond with those based on the geographical distribution of animal life. Europe, for instance, the most important of all the continents politically speaking, is for zoological geographers, as well as for physical, but a small fragment of Asia. Again, the strip of Africa which borders the Mediterranean and extends to the Sahara agrees closely, as regards its animal life, with Europe, and is altogether different from the great mass of the African continent. Proceeding to America we find that physical geographers, as well as political, divide the two great masses of the New World at Panama. But those who study distribution have ascertained that Central America and Southern Mexico belong zoologically to South America, and they are consequently obliged to place the line of demarcation much further north.
Let us, therefore, dismiss from our minds for the moment the ordinary notions of both physical and political
geography, and consider how the earth's surface may be most naturally divided into Primary Regions, taking the amount of similarity and dissimilarity of animal life as our sole guide. In order to endeavour to solve this problem, let us select the mammals, as the most highly organised and altogether the best-known group of the animal kingdom, and examine the geographical distribution of this class of animals over the world's surface.
Mammals are divided by naturalists into eleven large groups, called "Orders." As regards their distribution, however, these Orders fall into two very different categories, according as they live on land or in the water—terrestrial and marine. For out of the eleven Orders, one of the principal divisions of the Carnivora—the Pinnipeds or seals, and two other Orders in their entirety—(the Cetaceans or whales, and the Sirenians or Manatees) are specially adapted for existence in water. Land is, therefore, a barrier to their extension, whereas, on the contrary, in the case of the ordinary terrestrial mammals, land is the means by which they extend their ranges, and seas and rivers form their restraining boundaries.
We will for the present put aside the marine mammals, and address ourselves to the discussion of the distribution of the nine terrestrial Orders, namely:—
Now, as is generally agreed by naturalists, one of the most certain and best ascertained points in the classification of mammals is, that these nine Orders can be grouped
primarily in three natural divisions (which may, in fact, be considered as Sub-classes) of nearly equal value. These three Sub-classes are, as named by Professor Huxley—the Prototheria, embracing only the Order Monotremata— the Metatheria, equivalent to the Order Marsupialia, and the Eutheria, which includes all the remaining Orders from the Edentata to the Primates. Let us, therefore, consider the distribution of the members of these three Sub-classes on the earth's surface. When we come to examine the ranges of these groups on the map, we shall find that the Monotremes are wholly confined to Australia and New Guinea; that the Marsupials predominate in Australia, and are only met with elsewhere in South America (one or two species of Opossum occurring in North America, but being probably only recent intruders from the south); and that the typical mammals or Eutheria occupy the rest of the world.
Again, after examining the distribution of the seven Orders of typical mammals, we remark the following significant facts:—
1. The absence of Insectivores in South America.
2. The great prevalence of Edentates in the same country; the Sloths, Armadilloes, and Ant-eaters, constituting three out of the five known families of this Order, being entirely confined to South and Central America.
Taking these main facts as our guide, we may divide the land-surface of the Earth as follows, into three divisions:—
(1) Land where Marsupials prevail; no Eutherians except Rodents and Bats; Monotremes
Australia, New Guinea^