GENERAL CHARACTERISTICS 2691 



In the first place, we find that many marine fishes have a much less wide 

 geographical distribution than might at first sight be expected; while, 

 on the other hand, we find families and genera, and even species, of fresh-water fishes 

 inhabiting widely separated areas of the earth's surface. The primary division into 

 fresh-water and marine fishes does not form such a sharply defined boundary as is 

 commonly supposed, the transition being formed by the brackish-water types, spe- 

 cies or even individuals of which can accustom themselves to live in either salt or 

 fresh water. Then, again, we have certain essentially fresh- water fishes, like the 

 salmon and some kinds of catfish, which pass a certain period of their existence in 

 the ocean; while, on the other hand, some marine forms, such as sturgeons, period- 

 ically ascend rivers for the purpose of spawning. To a certain extent such habits 

 will help to explain the occurrence of peculiar families of fresh- water fishes (such 

 as the chromids of Africa, South America, and India) in widely-separated areas, 

 although this must probably be supplemented by dispersal from a common northern 

 centre. 



After the separation of the fresh-water and brackish -water types, the marine 

 fishes are divided by Dr. Gunther into a littoral, a pelagic, and a deep-sea group, 

 although here, again, no hard-and-fast lines can be drawn. The littoral or shore 

 fishes are those found in the immediate neighborhood of land or sunken shoals, the 

 majority living close to the surface, and very few descending as deep as three hun- 

 dred fathoms. Their distribution is determined not only by the temperature of the 

 surface water, but likewise by the nature of the neighboring land, and its animal 

 and vegetable products; some of these fishes being suitable to inhabit flat coasts 

 with muddy or sandy bottoms, while others frequent rock-bound shores where the 

 water is deep, and others, again, congregate round coral reefs. Cod, rays, and flat- 

 fish are well-known examples of this group. Pelagic fishes, such as tunnies, flying 

 fishes, swordfishes, and sunfishes, inhabit the superficial layers of the open ocean, 

 approaching the shores only by accident, or in some cases in search of food, or for 

 the purpose of spawning. Dr. Gunther writes that, "with regard to their distribu- 

 tion, they are still subject to the influences of light and the temperature of the 

 surface water; but they are independent of the variable local conditions which tie 

 the shore fish to its original home, and therefore roam freely over a space which 

 would take a fresh-water or shore fish thousands of years to cover in its gradual 

 dispersal. Such as are devoid of rapidity of motion are dispersed over similarly 

 large areas by the oceanic currents, more slowly than, but as surely as, the strong 

 swimmers. ' ' 



In marked contrast to the last are the deep-sea fishes, inhabiting the abyssal 

 depths of the ocean, where they are undisturbed by tides or currents, and live for 

 the most part in total darkness; their organization, in consequence of the great 

 pressure of the medium in which they live, preventing them from coming to the sur- 

 face in a healthy condition. From the similarity in the physical conditions of the 

 ocean depths in all parts of the world, there seems no reason why a single species of 

 deep-sea fish should not range from the Equator to the Poles; and the abyssal fauna 

 is probably more or less nearly the same throughout the globe. These fishes belong 

 for the most part to pelagic families, and especially to such types as are of nocturnal 



