Hooker imbibed from his father a passion for botanical research, and from his youth was inspired with a keen desire to indulge it by foreign travel. This was first gratified when Sir James Clark Ross [q. v.], a friend of his father, offered to take him, if he qualified in time, nominally as assistant surgeon, but actually as naturalist, on his own ship, the Erebus, on the Antarctic expedition. Thus Hooker, like Darwin and Huxley, 'began his scientific career on board one of Her Majesty's ships.' The filiation of Hooker's life-work to that of Darwin had an accidental origin. Charles Lyell of Kinnordy, father of Sir Charles Lyell [q. v.], had lent Hooker the proof-sheets of Darwin's 'Journal.' He was hurrying on with his studies and slept with them under his pillow to read at daybreak. They impressed him 'despairingly with the variety of acquirements, mental and physical, required in a naturalist who should follow in Darwin's footsteps.' He was casually introduced to Darwin in Trafalgar Square, and Lyell sent him a published copy of the 'Journal' on the eve of his departure. The Erebus sailed from Chatham on 29 Sept. 1839. Besides magnetic survey the collection of 'various objects of natural history' was 'enjoined to the officers.' There were three breaks in the voyage during southern winters, in Tasmania, New Zealand, and the Falklands, and these afforded Hooker ample opportunity for collecting.
On the return of the expedition in 1843 Hooker at once commenced the publication of the botanical results. They fill six quarto volumes (1844–60), with 2214 pages and 528 plates; two are devoted to the flora of the Antarctic Islands ('Flora Antarctica,' 1844-7), two to that of New Zealand (1852-4), and two of Tasmania (1855-60). The treasury made a grant of 1000l. to be expended on the plates. But beyond an honorarium of 350l. from each of the two colonies he received no remuneration.
Darwin had through the elder Lyell read the letters sent home by Hooker, and began a lifelong correspondence by warmly congratulating him on his return in December 1843. The intercourse of the two for the next fifteen years is a memorable page in scientific history. The permanence of species was substantially the belief with which Darwin, Hooker, and Huxley started on their expeditions. Fossil remains in South America convinced Darwin that the present inhabitants of a given area though similar were not identical with their predecessors in the pant; there had been an evolution in time. The animals and plants (worked out by Hooker in 1845-6) of the Galapagos, though related, differed in each island; the inevitable conclusion was that there had been an evolution in space. Species were clearly not permanent; and an explanation was needed. Hooker found that identical species occurred in islands 'separated by 3000 miles of ocean'; was it to be concluded, as Agassiz thought, that species had multiple origins ?
On 14 Jan. 1844 Darwin wrote to Hooker, 'I think I have found out the simple way by which species become exquisitely adapted to various ends.' This was natural selection; Hooker was the first to whom the theory was confided, and he read at the same time the first sketch of the 'Origin' (printed in 1909 by Mr. Francis Darwin). The confidence proved afterwards of no small importance. During the next fourteen years in which Darwin was occupied in elaborating his theory, he was almost in continuous correspondence with Hooker with regard to its details. 'The intimacy,' which began in 1843, 'ripened [on Hooker's side] into feelings as near to those of reverence for [Darwin's] life, work and character as is reasonable and proper' (L. L. ii. 20). Darwin for his part could write to him in 1862: 'For years I have looked to you as the man whose opinion I have valued more on any scientific subject than anyone else in the world ' (M. L. ii. 284). Writing to Lyell in 1866, Darwin said: 'his [Hooker's] mind is so acute and critical that I always expect to hear a torrent of objections to anything proposed; but he is so candid that he often comes round in a year or two' (M. L. ii. 138).
Darwin and Hooker were both ultimately inspired by Lyell. Darwin's problem was how species originate; Hooker's how they are distributed over the surface of the earth. If they worked on parallel lines, they mutually re-acted on one another, and Darwin saw clearly that the distribution problem was an essential feature in any evolutionary theory. Writing to Hooker in 1845, he said, 'I know I shall live to see you the first authority in Europe on that grand subject, that almost keystone of the laws of creation, geographical distribution' (L. L. i. 336).
In his 'Flora Antarctica' Hooker rejected emphatically the theory of 'multiple origins,' the supposition that the same species may have originated in more than one area. Darwin thought their occurrence
