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Huxley conscience of these latter days.' While he held doubt to be a duty, he had no tolerance for careless indifferentism ; and he was fond of quoting Goethe's description of a healthy active doubt: 'Eine thätige Skepsis ist die, welche unablässig bemüht ist, sich selbst zu überwinden.'

The fearless application of Cartesian criticism aroused great indignation between 1860 and 1870, but the essays and addresses published during this period did their work. They were certainly among the principal agents in winning a larger measure of tolerance for the critical examination of fundamental beliefs, and for the free expression of honest reverent doubt. The best evidence of the effect they have produced is the difficulty with which men of a younger generation realise the outcry caused by 'Man's Place in Nature,' or by the lecture 'On the Physical Basis of Life ' (ib. vol. i. 1868). Two passages from the last-named lecture may be quoted as giving a summary of Huxley's philosophical position in his own words :

'But if it is certain that we can have no knowledge of the nature of either matter or spirit, and that the notion of necessity is something illegitimately thrust into the perfectly legitimate conception of law, the materialistic position that there is nothing in the world but matter, force, and necessity, is as utterly devoid of justification as the most baseless of theological dogmas. The fundamental doctrines of materialism, like those of spiritualism and most other "-isms," lie outside "the limits of philosophical enquiry," and David Hume's great service to humanity is his irrefragable demonstration of what those limits are. . . . Why trouble ourselves about matters of which, however important they may be, we do know nothing and can know nothing ? We live in a world which is full of misery and ignorance, and the plain duty of each and all of us is to try to make the little corner he can influence somewhat less miserable and somewhat less ignorant than it was before he entered it. To do this effectually it is necessary to be fully possessed of only two beliefs the first, that the order of nature is ascertainable by our faculties to an extent which is practically unlimited ; the second, that our volition counts for something as a condition of the course of events. Each of these beliefs can be verified experimentally as often as we like to try. Each, therefore, stands upon the strongest foundation upon which any belief can rest, and forms one of our highest truths. If we find that the ascertainment of the order of nature is facilitated by using one terminology, or one set of symbols, rather than another, it is our clear duty to use the former ; and no harm can accrue so long as we bear in mind that we are dealing merely with terms and symbols.'

Those who 'care even more for freedom of thought than for mere advancement of knowledge' may well consider the effect produced by his lectures and essays upon the minds of English-speaking peoples to be the most important result of Huxley's work between 1860 and 1870. But they represent only a small part of the work he actually did during this period. He was an active member of four royal commissions (on the acts relating to trawling for herrings on the coast of Scotland, 1862 ; on the sea-fisheries of the United Kingdom, 1864-5; on the Royal College of Science for Ireland, 1866 ; on science and art instruction in Ireland, 1868). He was Hunterian professor at the Royal College of Surgeons from 1863 to 1869, and Fullerian professor at the Royal Institution from 1863 to 1867 ; he undertook an increasing amount of administrative work in connection with various learned societies, especially the Royal, the Zoological, and the Ethnological ; and he wrote frequently for the reviews, being himself for a short time an editor of the quarterly 'Natural History Review.' In spite of the increased demands upon his time and strength made by all these new duties, his purely scientific work rather increased than diminished in value and in amount.

The papers on fossil fishes, already referred to, were followed in 1861 by an 'Essay on the Classification of Devonian Fishes.' Apart from its great value as an addition to our knowledge of a difficult group of fishes, this essay is remarkable because in it Huxley drew attention to the type of fin which he called 'crossopterygian,' or fringed, because the fin-rays are borne on the sides of a longer or shorter central axis. The imperfect knowledge attainable from the study of fossils did not permit him at this time to describe the structure of the crossopterygium very fully ; but after the discovery of Ceratodus the conceptions foreshadowed in this essay acquired great importance in connection with attempts to find a common type of limb from which both the fin of an ordinary fish and the limb of an air-breathing vertebrate might conceivably have been derived.

In 1862 he delivered an address to the Geological Society, in which he attacked a doctrine then widely held. The order in which the various forms of life appear, as we examine the fossiliferous rocks from the oldest to the most recent, is practically the same in all parts of the world. This fact