Thomas Henry Huxley PC FRS (4 May 1825 – 29 June 1895) was an English biologist, known as Darwin's Bulldog for his advocacy of Charles Darwin's theory of evolution.

Huxley's famous 1860 debate with Samuel Wilberforce was a key moment in the wider acceptance of evolution, and in his own career. Huxley had been planning to leave Oxford on the previous day, but, after an encounter with Robert Chambers, the author of "Vestiges", he changed his mind and decided to join the debate. Wilberforce was coached by Richard Owen, against whom Huxley also debated whether humans were closely related to apes. Huxley was slow to accept some of Darwin's ideas, such as gradualism, and was undecided about natural selection, but despite this he was wholehearted in his public support of Darwin. He was instrumental in developing scientific education in Britain, and fought against the more extreme versions of religious tradition. Huxley used the term 'agnostic' to describe his own views on theology, a term whose use has continued to the present day.

Huxley had little formal schooling and taught himself almost everything he knew. Remarkably, he became perhaps the finest comparative anatomist of the latter 19th century. He worked on invertebrates, clarifying relationships between groups previously little understood. Later, he worked on vertebrates, especially on the relationship between apes and humans. After comparing Archaeopteryx with Compsognathus, he concluded that birds evolved from small carnivorous dinosaurs, a view widely held today. The tendency has been for this fine anatomical work to be overshadowed by his energetic and controversial activity in favour of evolution, and by his extensive public work on scientific education, both of which had significant effects on society in Britain and elsewhere.

Thomas Henry Huxley was born in Ealing, then a village in Middlesex. He was the second youngest of eight children of George Huxley and Rachel Withers. Like some other British scientists of the nineteenth century such as Alfred Russel Wallace, Huxley was brought up in a literate middle-class family which had fallen on hard times. His father was a mathematics teacher at Ealing School until it closed, putting the family into financial difficulties. As a result, Thomas left school at age 10, after only two years of formal schooling. One of Huxley's brothers, James Huxley (1821-1907), a medical practitioner, was a fairly well known Victorian psychiatrist, serving for many years as superintendent of Kent County Asylum, near Maidstone. Despite this unenviable start, Huxley was determined to educate himself. He became one of the great autodidacts of the nineteenth century. At first he read Thomas Carlyle, James Hutton's Geology, Hamilton's Logic. In his teens he taught himself German, eventually becoming fluent and used by Charles Darwin as a translator of scientific material in German. He learnt Latin, and enough Greek to read Aristotle in the original. Later on, as a young adult, he made himself an expert first on invertebrates, and later on vertebrates, all self-taught. He was skilled in drawing, and did many of the illustrations for his publications on marine invertebrates. In his later debates and writing on science and religion his grasp of theology was better than most of his clerical opponents. So, a boy who left school at ten became one of the most knowledgeable men in Britain.

He was apprenticed for short periods to several medical practitioners: at 13 to his brother-in-law John Cooke in Coventry, who passed him on to Thomas Chandler, notable for his experiments using mesmerism for medical purposes. Chandler's practice was in London's Rotherhithe amidst the squalor endured by the Dickensian poor. Here Thomas would have seen poverty, crime and rampant disease at its worst. Next, another brother-in-law took him on: John Salt, his eldest sister's husband. Now 16, Huxley entered Sydenham College (behind University College Hospital), a cut-price anatomy school whose founder Marshall Hall discovered the reflex arc. All this time Huxley continued his program of reading, which more than made up for his lack of formal schooling.

A year later, buoyed by excellent results and a silver medal prize in the Apothecaries' yearly competition, Huxley was admitted to study at Charing Cross Hospital, where he obtained a small scholarship. At Charing Cross, he was taught by the remarkable Scot, Thomas Wharton Jones, who had been Robert Knox's assistant when Knox bought cadavers from Burke and Hare. The young Wharton Jones, who acted as go-between, was exonerated of crime, but thought it best to leave Scotland. He was a fine teacher, up-to-date in physiology and also an ophthalmic surgeon. In 1845, under Wharton Jones' guidance, Huxley published his first scientific paper demonstrating the existence of a hitherto unrecognized layer in the inner sheath of hairs, a layer that has been known since as Huxley's layer. No doubt remembering this, and of course knowing his merit, later in life Huxley organised a pension for his old tutor.

At twenty he passed his First M.B. examination at the University of London, winning the gold medal for anatomy and physiology. However, he did not present himself for the final (2nd M.B.) exams and consequently did not qualify with a university degree. His apprenticeships and exam results formed a sufficient basis for his application to the Royal Navy. Aged 20, Huxley was too young to apply to the Royal College of Surgeons for a licence to practice, yet he was 'deep in debt'. So, at a friend's suggestion, he applied for an appointment in the Royal Navy. He had references on character and certificates showing the time spent on his apprenticeship and on requirements such as dissection and pharmacy. Sir William Burnett, the Physician General of the Navy, interviewed him and arranged for the College of Surgeons to test his competence (by means of aviva voce). Finally Huxley was made Assistant Surgeon ('surgeon's mate') to HMS Rattlesnake, about to start for a voyage of discovery and surveying to New Guinea and Australia. Rattlesnake left England on 3 December 1846 and, once they had arrived in the southern hemisphere, Huxley devoted his time to the study of marine invertebrates. He began to send details of his discoveries back to England, where publication was arranged by Edward Forbes FRS (who had also been a pupil of Knox). Both before and after the voyage Forbes was something of a mentor to Huxley.

Huxley's paper On the anatomy and the affinities of the family of Medusae was published in 1849 by the Royal Society in its Philosophical Transactions. Huxley united the Hydroid and Sertularian polyps with the Medusae to form a class to which he subsequently gave the name of Hydrozoa. The connection he made was that all the members of the class consisted of two cell layers, enclosing a central cavity or stomach. This is characteristic of the phylum now called the Cnidaria. He compared this feature to the serous and mucous structures of embryos of higher animals. When at last he got a grant from the Royal Society for the printing of plates, Huxley was able to summarise this work in The Oceanic Hydrozoa, published by the Ray Society in 1859.

The value of Huxley's work was recognized and, on returning to England in 1850, he was elected a Fellow of the Royal Society. In the following year, at the age of twenty-six, he not only received the Royal Society Medal but was also elected to the Council. He met Joseph Dalton Hooker and John Tyndall, who remained his lifelong friends. The Admiralty retained him as a nominal assistant-surgeon, so he might work on the specimens he collected and the observations he made during the voyage of Rattlesnake. He solved the problem of Appendicularia, whose place in the animal kingdom Johannes Peter Müller had found himself wholly unable to assign. It and the Ascidians are both, as Huxley showed, tunicates, today regarded as a sister group to the vertebrates in the phylum Chordata. Other papers on the morphology of the cephalopods and on brachiopods and rotifers are also noteworthy. The Rattlesnake's official naturalist, John MacGillivray, did some work on botany, and proved surprisingly good at notating Australian aboriginal languages. He wrote up the voyage in the standard Victorian two volume format.

Huxley effectively resigned from the navy (by refusing to return to active service) and, in July 1854, he became Professor of Natural History at the Royal School of Mines and naturalist to the Geological Survey in the following year. In addition, he was Fullerian Professor at the Royal Institution 1855–58 and 1865–67; Hunterian Professor at the Royal College of Surgeons 1863–69; President of the British Association for the Advancement of Science 1869–1870; and, later, President of the Royal Society 1883–85; and Inspector of Fisheries 1881–85. The thirty-one years during which Huxley occupied the chair of natural history at the Royal School of Mines included work on vertebrate palaeontology and on many projects to advance the place of science in British life. Huxley retired in 1885, after a bout of depressive illness which started in 1884. He resigned the Presidency of the Royal Society in mid-term, the Inspectorship of Fisheries, and his chair (as soon as he decently could) and took six month's leave. His pension was a fairly handsome £1500 a year.

In 1890, he moved from London to Eastbourne where he edited the nine volumes of his Collected Essays. In 1894 he heard of the Eugene Dubois' discovery in Java of the remains of Pithecanthropus erectus (now known as Homo erectus). Finally, in 1895, he died of a heart attack (after contracting influenza and pneumonia), and was buried in North London at St Marylebone. This small family plot had been purchased upon the death of his beloved youngest son Noel, who died of scarlet fever in 1860; Huxley's wife is also buried there. No invitations were sent out, but two hundred people turned up for the ceremony; they included Hooker, Flower, Foster, Lankester, Joseph Lister and, apparently, Henry James.

From 1870 onwards, Huxley was to some extent drawn away from scientific research by the claims of public duty. He served on eight Royal Commissions, from 1862 to 1884. From 1871–80 he was a Secretary of the Royal Society and from 1883–85 he was President. He was President of the Geological Society from 1868–70. In 1870, he was President of the British Association at Liverpool and, in the same year was elected a member of the newly-constituted London School Board. He was the leading person amongst those who reformed the Royal Society, persuaded government about science, and established scientific education in British schools and universities. Before him, science was mostly a gentleman's occupation; after him, science was a profession.

Huxley was for about thirty years evolution's most effective advocate. Though he had many admirers and disciples, his retirement and later death left British zoology somewhat bereft of leadership. He had, directly or indirectly, guided the careers and appointments of the next generation, but none were of his stature. The loss of Francis Balfour in 1882, climbing the Alps just after he was appointed to a chair at Cambridge, was a tragedy.

The first half of Huxley's career as a palaeontologist is marked by a rather strange predilection for 'persistent types', in which he seemed to argue that evolutionary advancement (in the sense of major new groups of animals and plants) was rare or absent in the Phanerozoic. In the same vein, he tended to push the origin of major groups such as birds and mammals back into the Palaeozoic era, and to claim that no order of plants had ever gone extinct. Much paper has been consumed by historians of science ruminating on this strange and somewhat unclear idea. Huxley was wrong to pitch the loss of orders in the Phanerozoic as low as 7%, and he did not estimate the number of new orders which evolved. Persistent types sat rather uncomfortably next to Darwin's more fluid ideas; despite his intelligence, it took Huxley a surprisingly long time to appreciate some of the implications of evolution. However, gradually Huxley moved away from this conservative style of thinking as his understanding of palaeontology, and the discipline itself, developed.

Huxley's detailed anatomical work was, as always, first-rate and productive. His work on fossil fish shows his distinctive approach: whereas pre-Darwinian naturalists collected, identified and classified, Huxley worked mainly to reveal the evolutionary relationships between groups. The lobed-finned fish (such as coelacanths and lung fish) have paired appendages whose internal skeleton is attached to the shoulder or pelvis by a single bone, the humerus or femur. His interest in these fish brought him close to the origin of tetrapods, one of the most important areas of vertebrate palaeontology. The study of fossil reptiles led to his demonstrating the fundamental affinity of birds and reptiles, which he united under the title of Sauropsida. His papers on Archaeopteryx and the origin of birds were of great interest then and still are.

Apart from his interest in persuading the world that man was a primate, and had descended from the same stock as the apes, Huxley did little work on mammals, with one exception. On his tour of America Huxley was shown the remarkable series of fossil horses, discovered by O.C. Marsh, in Yale's Peabody Museum. Marsh was part palaeontologist, part robber baron, a man who had hunted buffalo and met Red Cloud (in 1874). Funded by his uncle George Peabody, Marsh had made some remarkable discoveries: the huge Cretaceous aquatic bird Hesperornis, and the dinosaur footprints along the Connecticut River were worth the trip by themselves, but the horse fossils were really special. The collection at that time went from the small four-toed forest-dwelling Orohippus from the Eocene through three-toed species such as Miohippus to species more like the modern horse. By looking at their teeth he could see that, as the size grew larger and the toes reduced, the teeth changed from those of a browser to those of a grazer. All such changes could be explained by a general alteration in habitat from forest to grassland. And, we now know, that is what did happen over large areas of North America from the Eocene to the Pleistocene: the ultimate causative agent was global temperature reduction. The modern account of the evolution of the horse has many other members, and the overall appearance of the tree of descent is more like a bush than a straight line.

The horse series also strongly suggested that the process was gradual, and that the origin of the modern horse lay in North America, not in Eurasia. And if so, then something must have happened to horses in North America, since none were there when the Spanish arrived... That, however, is another story.