Thomas Henry Huxley (1825-1895) was a British biologist, best known for his active support of Charles Darwin's theory of evolution (he carried the nickname of "Darwin’s bulldog"). He received his medical degree from the University of London in 1845 and was admitted to the Royal College of Surgeons. The following year he entered the Royal Navy as assistant surgeon of the HMS Rattlesnake.

During his tour of duty in Australasian waters, which lasted until 1850, Huxley became thoroughly familiar with the surface animals of tropical seas. His observations on the medusa family of jellyfish led to the formulation of the zoological class Hydrozoa and to the realization that the two germ layers found in members of this class are comparable to the two germ layers that arise in the early embryological stages of higher animals. From this, he concluded tha investigating the adult forms of animals is not sufficient to conclusively prove that two species are similar. He realized that it was necessary to look at the entire development of the animals, from conception onward, to make a proper judgement.

Returning to England in 1850, Huxley was made a fellow of the Royal Society. The Royal Navy retained him as a nominal assistant surgeon until 1853; he used this time to write several scientific papers, including an authoritative work on the morphology of cephalopod mollusks. Huxley became professor of natural history and paleontology at the Royal School of Mines, London, in 1854. He accompanied the Irish physicist John Tyndall on an expedition to the Alps, where they studied glaciation.

When Charles Darwin published The Origin of Species in 1859, Huxley became the foremost supporter in England of Darwin's theory. His lucid, popular lectures on organic evolution, which he gave at various times from 1860 until his death, were an important factor in the acceptance of the theory of evolution by both scientists and the public. Huxley died in Eastbourne, Sussex, on June 29, 1895.

His writings include:

Other famous men on the Huxley family include Thomas Huxley's grandchildren Julian Huxley and Aldous Huxley.