little companies, and many in flocks of several thousand, with a chief and often in a definite order. The structure of these aggregates of animals relates evidently not only to the action of climates, winds, etc., but also to the abundance or scarcity of the alimentary resources at their disposal, to the exterior dangers with which they are menaced, to the instincts, and to the habits born of these repeated experiences.
There are cosmopolitan species such as the common crow which endures equally heat and cold, from the Cape of Good Hope to Greenland, and from the Gulf of Mexico to Hudson's Bay. One meets it wherever it finds its normal food. Those of the North, like the human species of the same countries, are distinguished from those of the South only by an extreme voracity.
There are, however, some sedentary species of birds, particularly in certain regions. Such are those which through selection and adaptation have in a constant manner acquired the faculty of equilibrating their peculiar constitution and the necessity of the conservation of the species with the geographical, climatic, and alimentary environment. They never migrate because they have no interest in displacing themselves.
The distribution of the terrestrial mammals is naturally more fixed. It is also historically more recent. The natural classification of the several groups of fossils shows us, indeed, the succession of animal forms in the geological strata according to the order of their relative superiority: first, fish; second, amphibians; third, reptiles; fourth, birds; fifth, mammals. This historic classification is confirmed by the parallelism of the appearance of the same forms with the law of succession of the states through which the embryos of the superior types pass.
In area, geographically limited as it is today, the arctic fauna differs from the temperate fauna, the latter from the tropical, and even the northern temperate fauna from that of the southern temperate zone. Furthermore, in this latter, one finds two regions—that of central Africa and that of South America. These are dissimilar in their mammals as well as in their birds, their reptiles, their mollusks, and their insects. Europe itself has some distinct mammiferous regions.
