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We recently sailed with our research ship through the Red Sea to the Indian Ocean where we did research work for about a year. Our main objectives (目标) were the little-known Maldive Islands. The Maldives consist of several thousand little islands, some so small that they are hardly a mile in diameter, some as large as twenty or thirty miles across. All the islands in the Maldives have developed on coral reefs (珊瑚礁). A coral reef is a flower garden of stone, growing like a wall or like a tower from the depths, and filled with the most confusing and most colourful varieties of life. The colours are very beautiful, especially in the shallow region down to sixty feet, where some of the red and yellow light of the sun’s rays still penetrates. But most corals are greenish, brown, bluish or yellow. Coral reefs offer shelter and food to thousands of creatures. The warmer the climate, the greater the variety of species there is. In the tropics, it is easier to find, in one place, ten different species than ten specimens of the same species, and a coral reef provides ample proof of that rule. Living conditions are most favourable so that many forms are able to develop and survive in their struggle for existence. Here is wide field for the diving biologist. The value of aqualung (水中呼吸器) diving for the biologist is that he can watch and collect on the spot. If men have not until recently ventured diving in the tropical seas, this has been mainly because of sharks. In the last twenty years I have encountered most of the really erous varieties of shark. There is always a certain risk, but, in general, sharks are much less erous than people think. You must not show fear or swim quickly away when they come into view. This attracts them, just as a cat is attracted by something on a string. Keep quiet, and, if the shark really heads towards you, then you can scare him off by swimming towards him yourself.

Large varieties of creatures can be found around a coral reef because ______ .