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 verieties of life. The colours are beautiful, especially in the shallow region down to sixty feet, where some if 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 climet, 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 existance.

The main subject of the passage is__________.