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 suj:>plemented 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 
