Darwin continued to research and extensively revise his theory while focusing on his main work of publishing the scientific results of the Beagle voyage. He tentatively wrote of his ideas to Lyell in January 1842; then in June he roughed out a 35-page "Pencil Sketch" of his theory. Darwin began correspondence about his theorising with the botanist Joseph Dalton Hooker in January 1844, and by July had rounded out his "sketch" into a 230-page "Essay", to be expanded with his research results and published if he died prematurely.

What was the name of Darwin's 35-page writing of his theory?