The society has numbered many prominent scientists amongst its fellows. One such was the botanist Robert Brown, who was Librarian, and later President (1849-1853); he named the cell nucleus and discovered Brownian motion. In 1854 Charles Darwin was elected a fellow; he is undoubtedly the most illustrious scientist ever to appear on the membership rolls of the society. Another famous fellow was biologist Thomas Huxley, who would later gain the nickname "Darwin's bulldog" for his outspoken defence of Darwin and evolution. Men notable in other walks of life have also been fellows of the society, including the physician Edward Jenner, pioneer of vaccination, the Arctic explorers Sir John Franklin and Sir James Clark Ross, colonial administrator and founder of Singapore, Sir Thomas Stamford Raffles and Prime Minister of Britain, Lord Aberdeen.