DISTRIBUTION. 323 



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, species 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 cat-fish, which pass a certain 

 period of their existence in the ocean ; while, on the other hand, some marine 

 forms, such as sturgeons, periodically 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 neighbourhood of land or sunken shoals ; 

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

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

 the surface-water, but likewise by the nature of the neighbouring land, and its 

 animal and vegetable products ; some of these fishes being suited 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-fish, sword-fishes, and sun-fishes, 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 distribution, they are still subject to the influences of light and the tempera- 

 ture 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 organisation, in consequence of the great 

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

 surface 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 habits ; and are characterised by their generally black or 

 silvery colour, although in a few instances the fin-rays and certain filaments are 



