We saw in the case of forests that hot climates conduce to a great variety of species, while in colder climates the species are few, but the number of individuals very great. Something of the same sort seems to occur with fishes. In warm seas the number of species is very great, while in colder seas there are fewer species, but those which do occur are sometimes found in vast numbers. Fortunately for man these prolific northern species are often edible, whereas in warm seas poisonous or inedible forms are common. The valuable cod family is found chiefly in high latitudes. The consequence of the facts just described is that valuable fisheries tend to occur in cool or cold climates rather than hot ones, and because of the dependence of so many forms on the Continental Shelf, they occur in the northern or land hemisphere rather than in the southern or oceanic one. The most valuable fisheries in the world seem to be those off Newfoundland, where the broad Continental Shelf, forming the so-called "banks," feeds myriads of cod. The mingling of the waters brought by the cold Labrador currents with those brought by the warm Gulf Stream perhaps influences this marvellous abundance of fish, as does also the waste brought by the icebergs. Next to the banks of Newfoundland the most valuable fishing ground is the shallow North Sea, which, as we have seen, lies on the surface of the Continental Shelf. Fish are much more abundant here than on the narrower shelf on the western coast of Britain, and the wealth of the North Sea has been an important factor in the development of the countries bordering it. The warm, salt, relatively deep, and tide-less Mediterranean is not nearly so rich in food fishes as the more northerly seas, a fact reflected in the large importation of dried fish alike from Newfoundland and from the region of the North Sea. But this is an economic and not a zoological statement, for the Mediterranean is in reality richer in fish species than the North Sea, in this respect, as in some others, approaching tropical regions. Among the economically important fish are the tunny, a very large form allied to the mackerel, which is dried, and sardines and anchovies, which are preserved in oil. Otherwise the fish are eaten fresh, and do not enter into general trade. Fresh-water fish are abundant all over Europe, but with some exceptions they are not greatly prized in those countries where the better-flavoured marine fish can be obtained. Elsewhere, as in Russia, Germany, and parts of France, they become important.