As exciting as this sounds, research by Dr. John van Wyhe, a historian of science at the University of Cambridge, shows that there is no evidence that Darwin delayed publishing his ideas for any other reason than because he was busy with other projects. Darwin spent ten years writing up and publishing his notes and observations from the Beagle voyage, producing five books. He then spent eight years working on his monograph of living and fossil barnacles. At the same time he had a growing family, unstable health, had to manage his finances and he also took a great interest in helping his local community. Darwin did not have the time to finish his work on evolution during these years, though he continued to read, make notes and think about the subject.
Darwin’s early sketches were also not his final word on evolution. During the 20 years during which he developed his theories before the publication of The Origin Darwin refined and extended his ideas as he came across new and more information. In addition Darwin put a large emphasis on collecting evidence to support his theory; this required time.
It is also not true that Darwin kept his theory a secret. He discussed it openly in letters with many of his friends and colleagues, and sent off many more letters asking various experts questions relating to his developing theory.
Darwin was a remarkable scientist, both in terms of the quality and quantity of work he produced. He was also not a one trick pony; natural selection was one of many contributions Darwin made to science. Given how busy he was, how hard he worked, how much information he gathered and how many experiments he performed in order to develop and support his theory the delay in publication is quite understandable. As Darwin’s friend, the Harvard botanist, Asa Gray put it 'I do not think 20 years too much time to produce such a book in'.