Darwin first heard about Malthus in letters from his sister Fanny, while he was voyaging on HMS "Beagle". Fanny told him Malthus' ideas were being promoted by Harriet Martineau, an early feminist writer. Later on, back in London, Darwin met Martineau over dinner, and put a lot of thought into the ideas of Malthus. The number of humans could double in 25 years. If they did not, it was because of competition for resources, such as food, or because wars and disease became more common.