While on the H.M.S. "Beagle", and later back home in London, Darwin had come across the ideas of the Rev. T.R. Malthus. Malthus had realised that, although humans could double their population every 25 years, it did not happen in practice. He thought the reason was that a struggle for existence (or resources) limited their numbers. If numbers increased, then famine, wars and diseases caused more deaths. Darwin, who knew that all living things could, in principle, increase their numbers, began to think about why some survived, while others did not. His answer took years to develop.