By December 1838 he had developed the principles of his theory.
At that time similar ideas brought others disgrace and association with the revolutionary mob. He was conscious of the need to answer all likely objections before publishing. While he continued with research, he had an immense amount of work in hand analysing and publishing findings from the Beagle expedition, and was repeatedly delayed by illness.
Natural history at that time was dominated by clerical naturalists whose income came from the Established Church of England and who saw the science of the day as revealing God’s plan.
Darwin found three close allies: Charles Lyell, Joseph Dalton Hooker and Thomas Huxley. Books by the eminent geologist Charles Lyell had influenced the young Darwin during the Voyage of the Beagle and he then befriended Darwin who he saw as a supporter of his ideas of gradual geological processes with continuing divine Creation of species.
By the 1840’s Darwin became friends with the young botanist Joseph Dalton Hooker, who had followed his father into that science, and after going on a survey voyage used his contacts to eventually find a position. In the 1850s Darwin met Thomas Huxley, an ambitious naturalist who had returned from a long survey trip but lacked the family wealth or contacts to find a career and who joined the progressive group around Herbert Spencer fighting to make science a profession, freed from the clerics.
(Darwin’s First Sketch of the Tree of Life)
Darwin made attempts to open discussions about his theory with his close scientific colleagues. In January 1842 Darwin sent a tentative description of his ideas in a letter to Lyell, then prepared a "Pencil Sketch" of his theory. He worked up his "Sketch" into an "Essay" in 1844, and eventually persuaded Hooker to read a copy in January 1847.
( Development of the Tree of Life from Darwin’s First Sketch)
By September 1854 Darwin’s other books reached a stage where he was able to turn his attention fully to Species, and from this point he was working to publish his theory.
In 1856 he was still bringing his friends round towards accepting evolution as a process, and was far from convincing them about the mechanism, but then Wallace’s entry into the discussion brought a new urgency to publication.