The breeding habits and migrations of the different species of whales are at present little understood. During the summer, when the water in the Polar circles swarms with certain varieties of pelagic crustacea, the whales congregate in these regions and are then most profitably hunted. At the end of the summer they appear to migrate towards warmer water nearer the Equator. They bring forth their young in warm, shallow water, and return to the whaling grounds in the spring. A young whale calf may be as much as 20 ft. long at birth. Whales were captured by the Norwegians over 1,000 years ago. In the Middle Ages—from the ninth to the seventeenth centuries—the Basques hunted the Black whale in the Bay of Biscay, and supplied Europe with oil and whalebone. Towards the end of the sixteenth century, as the Biscay whales became rare and more difficult to find, the whalers ventured further afield, and in 1612 discovered the Greenland whale. The Black or Biscay whale is now almost extinct, and there is every likelihood that the Greenland Right whale will also soon be exterminated. The capture of Sperm and Rorqual whales, although equally important, is a comparatively modern development. Modern whale fishing has become a very efficient art, owing largely to the invention of the shot-harpoon by a Norwegian, Sven Foyn, in 1870. This harpoon is discharged from a gun from the deck of a fast steamship. It penetrates the body of the whale in the vital region just behind the flipper. The invention of this weapon has made the killing of whales a matter of comparative ease and certainty. The inevitable consequence of this is that the whales are being killed in such large numbers that they are in danger of general extermination. Even before the introduction of the shot-harpoon, whales were being destroyed at an astonishing rate. Thus, during 40 years in the middle of the last century, over 300,000 whales were captured by the United States whale fisheries alone. The value of these whales was £65,000,000, so that each whale realized on an average £216. Of recent years—before 1914—a single large Greenland whale has realized as much as £900 for whalebone and £300 for oil. At the present time, over 20,000 whales are killed each year.