Final answer:
Alfred Russel Wallace's letter prompted Charles Darwin to publish his book on natural selection, after both had presented their ideas together at the Linnean Society.
Step-by-step explanation:
The scientist whose letter prompted Charles Darwin to publish his seminal work, On the Origin of Species by Means of Natural Selection, was Alfred Russel Wallace. Both Darwin and Wallace independently developed similar theories on natural selection based on their observations from expeditions around the world. In 1858, they presented their scientific papers on natural selection together at the Linnean Society in London. Darwin then published his comprehensive arguments for evolution by natural selection the following year.