Charles Robert Darwin (1809-1882), biologist who founded evolutionary biology by raising the concept of descent with modification from vague speculation to rigorous science. He proposed an important mechanism, natural selection, though he did not think that it was the exclusive mechanism of evolution. His published his Origin of Species when he learned that Alfred Russel Wallace had also thought of natural selection, caling it an "abstract" of a much larger tome that he one day hoped to write. He carefully avoided discussing the question of human evolution, instead treating it in some later books like The Descent of Man.
He performed naturalist duties in his famous Beagle voyage, which he later wrote at length about, and he did detailed studies of barnacles and orchids, showing how their various parts were modified in different ways to serve different purposes.
He started out as a rather orthodox Anglican who had wanted to become a country clergyman, but his religious beliefs gradually evolved toward agnosticism; in his biography, he wrote things like
I can indeed hardly see how anyone ought to wish Christianity to be true; for if so the plain language of the text seems to show that the men who do not believe, and this would include my Father, Brother and almost all my best friends, will be everlastingly punished. And this is a damnable doctrine. (p. 87)
In a letter that sold at auction in 2015, known as "the atheist letter" Charles Darwin said, "I am sorry to have to inform you that I do not believe in the Bible as a divine revelation & therefore not in Jesus Christ as the son of God." - Charles Darwin, November 24th, 1880