Explain the Malthus' Theory about population growth?

Darwin's Theory was also based on Thomas Malthus' writings on population growth, in which he explained that populations grow geometrically. For instance, a population may double every generation, whereas the food supply increases only arithmetically (that is, increases by a constant number - say 3 for instance - whereby 3 increases to 7, to 11, 15, and so on). If reproduction were allowed to continue unchecked, a population like elephants would cover the earth's surface before long. The fact that the earth isn't covered with elephants can be attributed to natural causes that limit the population growth. Natural causes such as death, disease, and limited resources serve to keep population levels down.