Darwin's Bulldog

... was an epithet applied not to Julian Huxley, but to his grandfather Thomas Henry Huxley, who lived from 1825 to 1895 (the years stated in the question). Thomas Huxley studied medicine at Charing Cross Hospital, but despite winning the gold medal for anatomy and physiology in his 'First MB' examinations he failed to present himself for the final (Second MB) exams. He consequently left without a university degree, and in 1840 he joined the Royal Navy; in 1846 he sailed on HMS Rattlesnake as Assistant Surgeon. This voyage gave him an opportunity to study marine invertebrates, and focused his interest in comparative anatomy. On returning to England in 1850 he was elected a Fellow of the Royal Society, and for the rest of his life he worked in various academic posts, including Professor of Natural History at the Royal School of Mines.

His other claim to fame is that he coined the term 'agnosticism'.

Thomas Huxley and his wife Henrietta had five daughters and three sons. Their second son was Leonard Huxley, who published biographies of several prominent 19th–century figures including Charles Darwin and Joseph Dalton Hooker, as well as his own father. He married Julia Arnold, grand–daughter of Thomas Arnold, the famous headmaster of Rugby School. They had three sons and a daughter; the sons included Aldous, who needs no introduction here, and Julian.

Julian Huxley lived from 1887 to 1975. He was secretary of the London Zoological Society from 1935 to 1942, the first Director of UNESCO, a founding member of the World Wildlife Fund, and the first President of the British Humanist Association.

Julia Huxley died of cancer in 1908, after which Leonard Huxley remarried and had two more sons. The younger son, Andrew Huxley, won the Nobel Prize in Physiology or Medicine in 1963, along with his colleague Alan Lloyd Hodgkin, for their discovery of the 'action potential', which is the basis for the propagation of nerve impulses.

To summarise: Thomas Henry Huxley ('Darwin's Bulldog') was the grandfather of Aldous Huxley, Julian Huxley, and the Nobel laureate Andrew Huxley. Aldous and Julian were brothers, and Andrew was their half–brother.