Mines from Jermyn Street to South Kensington gave the long-desired opportunity of completing his plan of instruction, by enabling every student to examine for himself, in the laboratory, the types described in the lectures. With the help of his four demonstrators, Thiselton Dyer, Michael Foster, Ray Lankester, and W. Rutherford, the course of laboratory work was perfected, and its main features are described in the well-known text-book of 'Elementary Biology' (1875), written in conjunction with Mr. H. N. Martin.
An important characteristic of Huxley's teaching, both in his lectures to students and in his technical memoirs, may here be noticed. Darwin had suggested an interpretation of the facts of embryology which led to the hope that a fuller knowledge of development might reveal the ancestral history of all the great groups of animals, at least in its main outlines. This hope was of service as a stimulus to research, but the attempt to interpret the phenomena observed led to speculations which were often fanciful and always incapable of verification. Huxley was keenly sensible of the danger attending the use of a hypothetical explanation, leading to conclusions which cannot be experimentally tested, and he carefully avoided it. This is well seen in the important essay on Ceratodus (1876), where a discussion of the way in which the iaws are suspended from the skull leads him to divide all fishes into three series. In one series the mode of suspension of the jaws is identical with that found in amphibia and the higher vertebrates ; and the hypothesis that these 'autostylic' fishes resemble the ancestors of air-breathing forms suggests itself at once. Although this was clearly present in Huxley's mind, he is careful to confine himself to a statement of demonstrable structural resemblance, which must remain true, whatever hypothesis of its origin may ultimately be found most useful. Again, in the preface to the 'Manual of the Comparative Anatomy of Invertebrated Animals' (1877) he says : 'I have abstained from discussing questions of ætiology, not because I underestimate their importance, or am insensible to the interest of the great problem of Evolution, but because, to my mind, the growing tendency to mix up ætiological speculations with morphological generalisations will, if unchecked, throw Biology into confusion.' The only attempts to trace the ancestry of particular forms which Huxley ever made are based on palæontological evidence, in the few cases in which the evidence seemed to him sufficiently complete. Such are the essays on the horse (Presidential Address to the Geological Society, 1870; American Addresses, 1876; Collected Essays, vols. iii. and viii.), and that on the 'Classification of the Mammalia' (Proc. Zool. Soc. 1880). The treatise on the crayfish (1879) may be taken as a statement of his mature convictions ; and the discussion of the evolution of crayfishes, given in this work, relates solely to the evidence of their modification since liassic times, which is afforded by fossils.
In 1870 the school board for London was instituted, and Huxley's interest in the problem of education led him to become one of its first members. In an essay on the first duties of the board (Contemporary Review, 1870 ; Collected Essays, vol. iii.) he lays stress on the primary importance of physical and moral culture. 'The engagement of the affections in favour of that particular line of conduct which we call good,' he says, 'seems to me to be something quite beyond mere science. And I cannot but think that it, together with the awe and reverence which have no kinship with base fear, but arise whenever one tries to pierce below the surface of things, whether they be material or spiritual, constitutes all that has any unchangeable reality in religion.' This feeling can, in his judgment, be best cultivated by a study of the Bible 'with such grammatical, geographical, and historical explanations by a lay teacher as may be needful.' He held that the elements of physical science, with drawing, modelling, and singing, afforded the best means of intellectual training in such schools. Huxley's influence upon the scheme of education finally adopted was very great, although he left the board in 1872.
In speaking of the later stages of education, he dwelt upon the great value of literary training as a means of intellectual culture, but he never tired of contending that a perfect culture, which should 'supply a complete theory of life, based upon a clear knowledge alike of its possibilities and of its limitations,' could not be acquired without a training in the methods of physical science. At the same time he was careful to emphasise his horror of the prevalent idea that a mere acquaintance with the 'useful' results of scientific work has any educational value. He well knew that educational discipline can only be obtained by the pursuit of knowledge without regard to its practical applications ; and he saw the need for sharply separating such educational discipline from the preparation for a handicraft or profession. Writing in 1893 to
