Darwin was spurred to finally publish his theory of natural selection by the writings of C. WALLACE
Darwin was writing up his theory in 1858 when he received an essay from Alfred Russel Wallace, who was in Borneo, describing Wallace's own theory of natural selection. Joint publication of Darwin's essay together with Wallace's paper was immediately prompted but received little notice from the public.
The public's disregard of their work spurred Darwin to write an abstract of his work which was published in 1859 as his book "On the Origin of Species".