In 1831, Charles Darwin was visiting Maer Hall, home of his uncle Josiah Wedgwood II, whom he told of his father, Robert Darwin's, opposition to him joining a two-year voyage on H.M.S. Beagle. Charles was enthusiastic about the opportunity, but his father objected to it as a waste of time, delaying his expected career in the clergy, and would not give his permission. His father said, however, he could be swayed to change his opinion if Charles found a man with common sense who would regard the proposed trip as being worthwhile.
Charles found that man in his uncle Josiah, who wrote a letter answering all of the objections in his favour. Josiah was Robert's brother-in-law, and as a family member was successful in influencing him to change his mind. From Today In Science History