Darwin had developed his ideas about sexual selection for this reason since at least the 1850s, and had originally intended to include a long section on the theory in his large, unpublished book on species. When it came to writing "Origin" (his "abstract" of the larger book), though, he did not feel he had sufficient space to engage in sexual selection to any strong degree, and included only three paragraphs devoted to the subject. Darwin considered sexual selection to be as much of a theoretical contribution of his as was his natural selection, and a substantial amount of "Descent" is devoted exclusively to this topic.