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Huxley to that time the evidence in favour of transmutation was wholly insufficient; and, secondly, that no suggestion respecting the causes of the transmutation assumed, which had been made, was in any way adequate to explain the phenomena.'

Darwin rendered a belief in the occurrence of transmutation far easier than it had been by his collection of facts illustrating the extent of variation; while the theory of natural selection provided a working hypothesis, adequate to explain the alleged phenomena, and capable of being experimentally tested. The attempt to secure a fair trial for the new hypothesis, which Huxley felt it his duty to make, involved a great expenditure of time and strength. The account of the 'Origin of Species' written for the 'Times' in 1859, and a lecture 'On Races, Species, and their Origin,' delivered in 1860, mark the beginning of a long effort, which only ceased as the need for it became gradually less. Many were the discussions of this doctrine in which he took part, and especially important and interesting was his share in the debate on the question during the meeting of the British Association for the Advancement of Science at Oxford in 1860.

The consequence of Darwin's theory, which many persons found the greatest difficulty in accepting, was a belief in the gradual evolution of man from some lower form; and evidence which seemed to establish a broad gap between the structure of man and that of other animals was welcomed. Great interest was therefore excited by a paper which Owen had read in 1857, and repeated with slight modification as the Kede lecture before the university of Cambridge in 1859. Owen declared that the human brain was distinguished from that of all other animals by the backward projection of the cerebral hemispheres, so as to cover the cerebellum, and by the backward prolongation of the cavity of each cerebral hemisphere into a 'posterior horn,' with an associated 'hippocampus minor.' It is difficult to understand how an anatomist of Owen's experience can have made these statements; and his subsequent explanations are equally unintelligible (e.g., Comparative Anatomy of Vertebrata, 1866, vol. i. pp. xix-xx). In 1861 Huxley published two essays, one 'On the Brain of Ateles Paniscus,' and one 'On the Zoological Relations of Man with the Lower Animals,' in which it was clearly shown that Owen's statements were inaccurate and inconsistent with well-known facts. Between 1859 and 1862 he gave a series of lectures 'On the Comparative Anatomy of Man and the Higher Apes,' published in book form under the title 'Zoological Evidences as to Man's Place in Nature' (1863, Collected Essays, vol. vii.) There is a sense in which the publication of this book marks the beginning of a new period of his work; because from the time of its appearance his writings attracted greater attention and affected a far greater number of people than before. This book and a series of lectures 'On the Causes of the Phenomena of Organic Nature,' addressed to working men and printed in 1863, were widely read and discussed, and from henceforth Huxley devoted a continually increasing amount of energy to popular teaching and to the controversy arising in connection with it. His sense of the importance of such work, and the enjoyment he derived from it, may be gathered from words which seem, although he uses them of Priestley, to give an admirable picture of himself. He says :

'It seems to have been Priestley's feeling that he was a man and a citizen before he was a philosopher, and that the duties of the two former positions are at least as imperative as those of the latter. However, there are men (and I think Priestley was one of them) to whom the satisfaction of throwing down a triumphant fallacy is at least as great as that which attends the discovery of a new truth, who feel better satisfied with the government of the world when they have been helping Providence by knocking an imposture on the head, and who care even more for freedom of thought than for mere advancement of knowledge. These men are the Carnots who organise victory for truth, and they are at least as important as the generals who visibly fight her battles in the field ' (1874, Collected Essays, vol. iii.)

The freedom of thought for which Huxley contended was freedom to approach any problem whatever in the manner advocated by Descartes; and he wishes his more important essays to be regarded as setting forth 'the results which, in my judgment, are attained by an application of the "method" of Descartes to the investigation of problems of widely different kinds, in the right solution of which we are all deeply interested' (ib. vol. i. preface). In 1870, after describing Descartes's condition of assent to any proposition, he says: 'The enunciation of this great first commandment of science consecrated doubt. It removed doubt from the seat of penance among the grievous sins to which it had long been condemned, and enthroned it in that high place among the primary duties which is assigned to it by the