In my opinion, Jean-Baptiste Lamarck had the greatest influence on Darwin’s theory of natural selection, in both a positive and negative way.

Lamarck was a botanist and zoologist and was one of the initial few to suggest that humans evolved
from a lower species, and that this was done by species adapting over time. Lamarck theorized that
adaptations occurred in simple life forms spontaneously, and would disappear if those adaptations were not used. His theories inspired Darwin to investigate on a positive level, as Darwin agreed with the overall theory, especially his explanations of vestigial structures, and in a negative way, as Darwin did not agree with Lamarck on how these changes occurred, disagreeing with the idea spontaneity being the way that the changes in species occurred. (https://evolution.berkeley.edu/evolibrary/article/history_09)

The points relating to Lamarck’s work in “How does evolution work” that most greatly affected Darwin’s work are “If the environment changes, the traits that are helpful or adaptive to that environment will be different”, “In order for traits to evolve and change, they must be heritable”, and Individuals do not evolve. Populations do.” As briefly mentioned above, Darwin worked on Lamarck’s existing theories but wanted to disprove part of the theory that he did not feel was correct, that adaptations occurred spontaneously. Darwin was able to disprove this through his theory of natural selection, which stated that the most successful organisms would be those whose traits adapted to their environment, due to their ability to obtain the resources they needed to reproduce and thrive, while those who were unable to do so would eventually cease to exist. Darwin also disproved Lamarck on the spontaneity theory by showing that those traits must be able to be passed on from generation to generation, although he was never able to completely explain how this occurred. Lastly, Darwin showed that since populations evolve, over great amounts of time, not over a single generation, that Lamarck’s theory of those changes spontaneously occurring was again incorrect for another reason as well.

While Darwin may still have developed his theory of natural selection without the influence of Lamarck’s work, it most likely would have taken much longer to arrive at his ultimate conclusions. The idea of evolution was already widely accepted in the scientific community at the time, and it is almost a certainty that Darwin was pursuing information in that area, but Lamarck’s theory was flawed in such a way that it was easy for Darwin to recognize and this led him to a much quicker path to natural selection (though the path itself was not quick by any stretch of the imagination).

Darwin, like most scientist of the time, was reticent to do anything that might cause himself harm from the church. The church had consistently taken action against anyone previously who had presented any scientific theory that was contrary to the religious tenants that the church espoused, and not wanting to risk those types of actions against himself (and because his wife was heavily involved in the church) Darwin ultimately chose not to publish his findings and they only came to light when, Alfred Russel Wallace, inspired him to actually publish “Origin of the Species” in 1859, 20+ years after his research, for fear that Wallace would get the credit for Darwin’s work after Wallace submitted some papers that reflected the same work Darwin had toiled over all those years prior.