* Alfred Russel Wallace: Wallace independently came up with a theory of evolution by natural selection, which he sent to Darwin in 1858. This spurred Darwin into action, as he realized his ideas were no longer unique and risked being scooped. The joint presentation of their theories at the Linnean Society in 1858 ultimately motivated Darwin to finally publish his work.
* Charles Lyell: Lyell was a close friend and mentor to Darwin. He strongly encouraged Darwin to publish his findings, arguing that the scientific community needed to see them. Lyell also helped Darwin prepare his manuscript for publication.
* Darwin's own conviction: While influenced by others, Darwin was ultimately driven by his own deep conviction in the validity of his theory. He spent over 20 years researching and developing his ideas, and was committed to sharing them with the world.
It's important to remember that Darwin's decision to publish was a complex one, influenced by a combination of factors, including personal ambition, scientific responsibility, and the fear of being preempted by Wallace.