DISTRIBUTION OF ANIMALS, or
. Knowledge of the geographical
distribution of animals, and their faunal relations
past and present, constitutes a very important
part of the science of zoölogy. An animal's
environment exerts a formative influence upon
its character and structure. Knowledge of its
habitat and that of its species, together with such
changes as the area and range may have undergone, is therefore essential to a full understanding of the natural history of any kind of animal or animals.
. Some animals are fitted to dwell wholly upon land, others in water. The aquatic animals are, further, divisible into those of salt and those of fresh waters. The former have the oceans of the world open to them, yet few, if any, range through all seas; furthermore, the inhabitants of the surface of the sea differ from those of its bottom or deeper parts. Similarly, animals able to mount into the air and to fly abroad with apparent freedom, are rarely cosmopolitan, and whole groups of these, as well as innumerable species and individuals, reside only within limited areas or along narrow lines of migratory travel. Fresh-water forms are likely to be restricted to separate lakes and river systems. Of the terrestrial animals none are now cosmopolitan, except those which have been carried by man to all parts of the world. (The introduced fauna of a region must always be distinguished from its indigenous fauna.) On the contrary, each of the present grand divisions of the earth has a characteristic fauna of its own, and often a whole family, order, or even subclass, belongs to one continent alone. Coincident with these restrictions and diversities, likenesses between widely separated lands are manifest; and representatives of a single group may be found in regions far distant from one another. To set forth these facts, to endeavor to account for them, and to develop their significance, is the province of zoögeography.
. Each kind of animal must have had a point of origin, whence it spread as it increased. Theoretically, the expansion of a species would proceed equally in all directions, but actually this expansion has encountered barriers and restrictions that have confined and shaped habitats within certain areas, in some instances extremely wide, in others surprisingly narrow. What constitutes these barriers and restrictions? That depends in each case on the physical surroundings as related to the needs and abilities of the animal in question. It is plain that a species of fish that originated in or somehow became restricted to an inland water—Lake Baikal, for example—could never spread beyond its shores save by accidental transplantation; while an able sea-going fish may wander indefinitely, so far as mere room is concerned. Among land animals space for expansion, then, is of prime importance. Here the firmest bounds are set by the sea. Any considerable breadth of water, and especially of salt water, is uncrossable except by rare and extraordinary accident; and even then a pregnant female or a pair must be landed on the further shore in order to start a colony, which must, furthermore, find favorable surroundings in order to survive in the new locality. This accounts for the fact that mammals and terrestrial reptiles and amphibians are absent from oceanic islands. Continuity of land, then, is necessary to the spread of a terrestrial species; and when species are found in regions now widely separated, as Europe and North America, it can usually be shown that such regions were formerly connected by lands now submerged. Conversely, the characteristics of the present fauna of such comparable regions assist the geologist to determine when the connection was finally broken.