Wallace deliberately planned some of his fieldwork to test the hypothesis that under an evolutionary scenario closely related species should inhabit neighbouring territories. During his work in the Amazon basin, he came to realise that geographical barriers—such as the Amazon and its major tributaries—often separated the ranges of closely allied species, and he included these observations in his 1853 paper "On the Monkeys of the Amazon". Near the end of the paper he asks the question, "Are very closely allied species ever separated by a wide interval of country?"