HORIZONTAL DISTRIBUTION OF MARINE ANIMAL LIFE The horizontal distribution of marine fauna is, like that of the land, dependent primarily upon climatic conditions. Thus the shark and sperm whale exist in the tropical and temperate seas, while the right whale frequents the colder northern seas. The herring, cod, and other edible fish abound in the temperate zones only ; while the walrus, rorqual, nar whal, and various kinds of seals, inhabit the icy-cold polar waters. Other kinds are peculiar to certain areas, as the tunny in the Mediterranean, &c. The various forms of marine animal life are generally limited as to their range, similarly to the fauna of the land ; but the greater facility of intercom munication throughout the ocean prevents such an arbitrary restriction as on land. Thus we find the right whale and the sperm whale frequenting the same ground, although the former is essentially a polar, and the latter a tropical, animal. Still, a broad division of the ocean into zones of generally similar forms of life is not only possible, but also convenient, and indeed necessary.
The homozoic zones into which Professor Forbes divided the ocean can only be simply enumerated here ; further information will be found in Mr. Forbes' " Natural History
of the European Seas." In the northern hemisphere the zones are ;—(1) The median zone, included within the tropics, in habited chiefly by essentially tropical forms. To the north of this zone is (2) the circumcentral zone, narrow and irregular, embracing a limited belt in the Atlantic north of N. lat., and including the Mediterranean, Black Sea, Caspian, and Sea of Aral, and continued across the Pacific, from Japan to Cali fornia. (3) The neutral zone includes a gradually widening belt of the Atlantic, from the American coast to the western shores of Great Britain, and continued in the North Sea and Baltic. The special species in this zone is the herring, while in the next zone—(4) the circumpolar (embracing the tracts in the northern sweep of the Gulf Stream, towards Iceland and Norway, and a similar belt in the Pacific)—the cod-fishery is mainly carried on. (5) Within the polar zone is included the Arctic seas, inhabited by the right whale, seals, and other Arctic marine fauna