Struggle for existence
While recovering from malaria in the East Indies, Wallace sent an essay to Darwin in 1858. It contained the phrase: "The life of wild animals is a struggle for existence". Wallace had not known much about Darwin's ideas, and reached the idea from exactly the same source: one of the later editions of Malthus' Essay on population. The arrival of this essay led to the joint publication of the idea of evolution by natural selection in 1958.