When did Darwin ‘clearly conceive’ his theory of evolution?

Darwin’s theory of evolution by natural selection as espoused in his classic book, On the Origin of Species, constitutes one of the most important scientific advances of the last millennium. In his autobiography, Darwin apparently claimed to have ‘clearly conceived’ the theory in 1839, but the Origin was not published until 1859, following a brief excerpt of his theorizing published in mid-1858. Much scholarship has been devoted to explaining this apparent gap. Yet, examination of Darwin’s explicit theorizing fails to support the generally accepted ‘clear conception’ by 1839 as an accurate reference to an immature version of the Origin theory, a theory of continuous and universal organic change. However, by articulating both broad and narrow versions of the theory, as well as considering the inevitable theory development over decades, Darwin’s ‘clear conception’ is explained in relation to two distinct broad theories and the enduring core element, the mechanism of natural selection. This explanation casts the famous ‘gap’ in an entirely new light.