A Surprise Challenger
Another British naturalist and explorer, Alfred Russel Wallace, devised his own theory of species diversity based on his travels in the Amazon and Malay archipelago. In 1858, he sent an essay on his work to Darwin, who realized their theories were essentially identical. Darwin then rushed to set his down on paper and it was published first.
Darwin jotted down many of his ideas and observations in a series of notebooks. He drew this “tree of life” in July 1837 to illustrate his concept of descent with modification — how one species could evolve into many — but did not formalize his theory for another 20 years.