FISHES, Geographical Distribution of. The laws governing the distribution of animals are reducible to three very simple propositions. Each species of animal is found in every part of the earth having conditions suitable for its maintenance, unless: (a) Its individuals have been unable to reach this region through -barriers of some sort; or, (b) Having reached it, the species is unable to maintain itself, through lack of capacity for adaptation, through severity of competition with other forms, or through destructive conditions of environment ; or else, (c) Having entered and maintained itself, it has become so altered in the process of adaptation as to become a species distinct from the original type.
Under the first head, numerous illustrations may be given. The absence of loaches, Nile fishes (Mormyrus) in America and of sun Fishes, suckers and mooneyes in Europe may serve as examples.
Of species under (b), those that have crossed the seas and not found lodgment, there is, in the nature of things, no record. Of the existence of multitudes of estrays there is abundant evidence. Now and then one among thousands establishes itself permanently,, and by such means a species from another region will he added to the fauna. The rest disappear and leave no trace. A knowledge of the currents of the sea and their influence is essential to any detailed study of the dispersion of fishes.
In the third class, that of species changed in the process of adaptation, most insular forms belong. As a matter of fact, at some time or another almost every species must be in this category, for isolation furnishes the most potent elements in the initiation and intensification of the minor differences which separate related species. It is not the preservation of the most useful features, but of those which actually ex isted in the ancestral individuals, which dis tinguished such species. In many cases the per sistence of characters rests not on any special usefulness or fitness, but on the fact that indi viduals possessing these characters have, at one time or another, invaded a certain area and populated it. In other words over and beyond the *Survival of the Fittest,* we have the *Sur vival of the Existing.' The former element is competitive, producing adaptation to conditions. The latter is non-competitive, maintaining hereditary traits, not necessarily useful in them selves.
Barriers Checking Movement of Fishes.— The limits of the distribution of individual species or genera must be found in some sort of barrier, past or present. The chief barriers which limit marine fishes are the presence of land, the existence of great oceans, the differ ences of temperature arising from differences in latitude, the nature of the sea-bottom and the direction of oceanic currents. That which is a barrier to one species may be an agent in dis tribution to another. The common shore-fishes would perish in deep waters almost as surely as on land, while the open Pacific is a broad highway to the albacore or the swordfish.
Again, that which is a barrier to rapid dis tribution may become an agent in the slow ex tension of the range of a species. The vast continent of Asia is undoubtedly one of the greatest of barriers to the wide movement of species of fish, yet its long shore-line enables species to creep, as it were, from bay to bay, or from rock to rock; till, in many cases, the same species is found in the Red Sea and in the tide pools or sand-reaches of Japan, or even in the brooks or coral pools of Tahiti or Samoa. In the North Pacific the presence of a range of half-submerged volcanoes, known as the Aleu tain and the Kurile islands, has greatly aided the slow movement of the fishes of the tide pools and the kelp. To a school of mackerel or of flying fishes such rough islands might form an insuperable barrier.
Temperature the Central It has long been recognized that the matter of tempera ture is the central fact in all problems of geo graphical distribution. Few species in any group freely cross the frost-line, and except as borne by oceanic currents, few species extend their range far into waters colder than those in which the species is distinctively at home. Knowing the average temperature of the water in a given region, we know in general the types of fishes which must inhabit it. It is the sim ilarity in temperature and physical conditions, not the former absence of barriers, which chiefly explains the analogy of the Japanese fauna to that of the Mediterranean or the Antilles. This fact alone must explain the resemblance of the Arctic and Antarctic fauns. Like forms lodge in like places.