  This vision influenced Darwin profoundly, as he freely admitted. While he became convinced by his observations and reading that the fossil record and current distribution of species could only be due to the gradual transformation of one species into another, he was determined to articulate a theory that measured up to Lyell’s principles. The crucial event in convincing him that this was to be his life’s work was likely a visit to Cape Town, South Africa during the Beagle’s return trip to England. John F. W. Herschel was in Cape Town on a mission to do for the Southern Hemisphere what his father William had done for the Northern, namely to develop a comprehensive star map with the new powerful telescopes developed by his father and aunt.  As noted earlier, Darwin had been deeply impressed by Herschel’s Preliminary Discourse on the Study of Natural Philosophy when it first appeared a year before the Beagle set sail, and in his private journal he referred to his meetings with Herschel during a week long stop in Cape Town in June of 1836 as among the most profound events of the entire voyage. Just five months before meeting Darwin, Herschel had finished reading the 2nd edition of Lyell’s Principles. He sent Lyell a long letter filled with detailed constructive commentary.  The letter opens by praising Lyell for facing the issue of the ‘introduction of new species’—which Herschel calls ‘that mystery of mysteries’—scientifically, and for advocating that we search for ‘intermediate causes’ to explain these ‘introductions’—code for natural, as opposed to ‘miraculous’,  causes.[5]   This part of the letter was quoted in Charles Babbage’s Bridgewater Treatise, published in 1837 while Darwin was struggling to develop just such a theory. Upon reading the Herschel quotation in Babbage, Darwin wrote in his private ‘species’ notebooks: 