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Show CHAPTER II. THE ELEMENTARY FACTS OF DISTRIBUTION. Importance of Locality as an essential character of Species-Areas of Distributio~- Extent and Limitations of Specific Areas-Specific range of Birds-Generic Areas-Separate and overlapping areas-The species of Tits as illustrating Areas of Distribution-The distribution of tho species of Jays-Discontinuous generic areas-Peculiarities of generic and family distribution-General features of overlapping and discontinuous areas-Restricted areas of Families-The distribution of Orders. So long as it was believed that the several species of animals and plants were " special creations," and had been formed expressly to inhabit the countries in which they are now found t~eir habitat was an ultimate fact which required no cxplana~ twn. I~ was assumed that every animal was exactly adapted to the chmate and surroundings amid which it lived, and that ~be o~ly, or, at all events, the chief reason why it did not mhab1t another country was, that the climate or general conditions of that country were not suitable to it, but in what the unsuitability consisted we could rarely hope to discover. Hence. the exact locality of any species was not thought of much Importance from a scientific point of view, and the idea t~at anything could be learnt by a comparative study of different floras and faunas never entered the minds of tho older naturalists. But so soon .as the theory of evolution came to be generally adopt~d, an~ It was seen that each anjmal could only have co~e mto .existence in. some area where ancestral forms closely .alhed to It already lived, a real and important relation was ciiAr. n.] THE ELEMENTARY FACTS OF DISTRIBUTION. 13 established between an animal and its native country, and a new set of problems at once sprang into existence. From the old point of view the diversities of animal life in the separate continents, even where physical conditions were almost identical, was the fact that excited astonishment ; but seen by the light of the evolution theory, it is the resemblances rather than the diversities in these distant continents and islands that are most difficult to explain. It thus comes to be admitted that a knowledge of tho exact area occupied by a species or a group is a real portion of its natural history, of as much importance as its habits, its structure, or its affinities; and that we can never arrive at any trustworthy conclusions as to how the present state of the organic world was brought about, until we have ascertained with some accuracy the general laws of the distribution of living things over the earth's surface. Areas of Distrib't6tion.-Every species of animal has a certain area of distribution to which, as a rule, it is permanently confined, although, no doubt, the limits of its range fluctuate somewhat from year to year, and in some exceptional cases may be considerably altered in a few years or centuries. Each species is moreover usually limited to one continuous area, over the whole of which it is more or less frequently to be met· with, but there are many partial exceptions to this rule. Some animals are so adapted to certain kinds of country-as to forests or marshes, mountains or deserts-that they cannot live long elsewhere. These may be found scattered over a wide area in suitable spots only, but can hardly on that account be said to have several distinct areas of distribution. As an example we may name the chamois, which lives only on high mountaine, but is found. in the P~renees, the Alps, the Carpathians, in some of the Greek mountams and the Caucasus. The variable hare is another and more remarkable case, being found all over Northern Europe and Asia beyond lat. 55°, and also in Scotland and Ireland. In Central Europe it is unknown till we come to the Alps, the Pyrenees, and the Caucasus, where it again appears. This is one of the best cases known of the discontinuous distribution of a species, there being a gap of about a thousand miles between its southern limits in Russia, and its reappearance in |