Charles Darwin was a naturalist, geologist and biologist, best known for his contributions to the science of evolution. His proposition that all species of life have descended over time from common ancestors is now widely accepted, and considered a foundational concept in science.
While he seemed to recognise the moral worth of sentient non-humans, there seems to be little basis to the suggestion that he was vegan or vegetarian, but his great-great grandson is confident he'd be vegan if he was alive today. Darwin had a naturalistic worldview and considered himself agnostic.
He said: "We have seen that the senses and intuitions, the various emotions and faculties, such as love, memory, attention and curiosity, imitation, reason, etc., of which man boasts, may be found in an incipient, or even sometimes in a well-developed condition, in the lower animals.", "There is no fundamental difference between man and the higher mammals in their mental faculties ... The difference in mind between man and the higher animals, great as it is, certainly is one of degree and not of kind", and "The love for all living creatures is the most noble attribute of man."
Darwin on Wikipedia