Burnet, Sir Macfarlane, muk FAHR luhn (1899-1985), an Australian physician and virologist, shared the 1960 Nobel Prize in physiology or medicine with British biologist Peter Brian Medawar. In 1949, Burnet and his colleague Frank Fenner proposed the theory of acquired immunological tolerance. They stated that an animal's immune system develops gradually during fetal life and if foreign tissues were introduced into the fetus early in its development, the fetus would accept the tissues as its own. Later, if tissues from the same foreign source were introduced into the adult animal, the immune system would not reject the tissues. Acquired immunological tolerance was later proved by Medawar and his colleagues.