What prompted darwin to publish his theory of evolution?

Darwin was reluctant to publish his ideas because they went so strongly against the ideas of the day. Darwin thought that the society was not yet ready for his ideas, and he knew that there would be a lot of protest.

What finally caused Darwin to publish this work was a letter from English naturalist Alfred Wallace, who knew that Darwin was interested in evolution. The letter came in June, 1858, from Malaya. With it was a summary of Wallace's theory, On the Tendency of Varieties to Depart Indefinitely from the Original Type. Darwin was amazed! This was his own theory written out by Wallace. He even said: "Even his terms stand as heads of my chapters."