In the memoir on the medusæ a comparison was made between the two cellular 'foundation layers' out of which the body wall and the various organs of a polyp or a medusa are formed, and the two primary layers recognised by Pander and Von Baer in the early embryos of vertebrates. Similarities between the adult condition of lower, and the embryonic condition of higher members of the same group of animals had been recognised by Meckel, and more fully by Von Baer ; but this comparison between the early embryo of the highest vertebrates and the adult condition of the simplest multicellular animals then known went far beyond any previous suggestion of the kind. This comparison paved the way for the attempts inaugurated later by Haeckel and Dr. Ray Lankester, under the influence of Darwin, to interpret the embryonic histories of the higher animals as evidence of their common descent from a two-layered ancestor, essentially like a hydroid polyp.
On his return to England in 1850 Huxley learnt that the value of his work on Medusæ had been fully recognised. He was elected F.R.S. in 1851, was granted the society's medal in 1852, and found the leading biologists in London, especially Edward Forbes, were anxious to help him. With their help, and that of Sir John Richardson, he obtained from the admiralty an appointment as assistant surgeon to a ship then stationed at Woolwich, with leave of absence which enabled him to arrange the materials amassed during his voyage, and to prepare his notes for publication. Accordingly in 1851 he published two memoirs on the Ascidians, in which several aberrant genera (especially appendicularia and doliolum) are shown to be modifications of the same morphological type as that found in other ascidians ; the relation between salpa and other ascidians is clearly explained, while the phenomenon of budding, alternating with sexual reproduction, which had been shown to occur by Chamisso and Eschscholtz, is fully described. In the paper 'On the Morphology of the Cephalous Mollusca' (Phil. Trans. 1853) a great advance is made upon all previous efforts to recognise the structural plan common to the various modifications of the 'foot,' and the structure of the pelagic 'heteropods' is described. These expositions of the morphology of three widely different groups of animals established Huxley's reputation as a scientific anatomist of the first rank; and the success which attended his use of simple inductive generalisation as a statement of morphological type had great effect upon the methods of English biologists. While winning reputation and the warm friendship of many among the ablest men in London, he was not earning money ; and without pecuniary help of some sort it was impossible even to publish some of his results. The admiralty felt unable to use funds, entrusted to it for other purposes, in assisting to publish anatomical works ; and not only so, but in January 1854 Huxley's request for further leave of absence was met by an order to join a ship at once. Rather than obey this order he preferred to leave the service, and with it his only certain income, determined to maintain himself somehow, by writing and lecturing, until he could gain an assured income without giving up all hope of scientific work. Fortunately a chance of doing this soon appeared. In June 1854 his friend, Edward Forbes, who had just commenced his course of lectures at the Royal School of Mines in Jermyn Street, was appointed to the professorship of natural history in Edinburgh. Huxley undertook to finish the course in London ; in July he was appointed lecturer on natural history at the Royal School of Mines, and naturalist to the geological survey in the following year. The salary attached to these posts was small, but with such additions as he could make to it in other ways he felt justified in taking an important step. During the visits of the Rattlesnake to Sydney, Huxley had met and won the affection of Miss H. A. Heathorn, and he felt that his position was now so secure that he might ask her to share it. Miss Heathorn and her parents set sail for England early in 1855, reaching London in May. The marriage took place in July of the same year.
Before the end of 1855 Huxley had published more than thirty technical papers, and he had given a number of lectures to unprofessional audiences. One of these, 'On the Educational Value of the Natural History Sciences' (1854, Collected Essays, vol. iii.), contains those statements concerning the fundamental unity of method in all sciences, the value of that method in the affairs of daily life, and its importance as a moral and intellectual discipline, which form the essence of his popular teaching in later years.
From 1855 until 1859 Huxley's time was largely occupied by the duties of his new post. In his teaching he quickly adopted a system afterwards developed until it became the model which teachers of biology throughout the country endeavoured to imitate. In his lectures he described a small series of animals, carefully chosen to illustrate im-
