the moon and the earth, with a consequent lengthening of the month. Tracing this effect back into the past Darwin arrived at a stage, 54 million years ago or more, at which the moon was only about 6,000 miles from the earth’s surface, while the two bodies rotated together, each always turning the same face to the other. The day and month, at this time equal, were each rather less than a quarter of our present day. He concluded that the earth and moon must originally have formed a single mass, and he was led to study the process by which this mass had broken up.
A further application of the theory of tidal friction to the motion of the planets round the sun opened up the wider question of the genesis of the solar system. Darwin had at first believed that tidal friction would account for the evolution of the whole solar system, but he subsequently adopted the view that tidal friction had been of primary importance only in the one case of the earth-moon system, which he consequently supposed to form a unique example in the solar system of this special method of evolution. At this time the generally accepted theory of the origin of the planets and their satellites was that propounded by Laplace, according to which each planet and satellite had been formed by the condensation of a ring of matter shed by the primary body around which it revolved. Darwin’s researches led him to contemplate the simpler possibility of an astronomical body breaking directly into two detached masses, and he tried to reconstruct the details of the process by tracing back the history of such a pair of bodies as our earth and moon still farther than had already been done. In the meantime Jules Henri Poincaré was attacking the same problem from the other end, examining the sequence of events in a mass which, owing to continued shrinkage, was rotating so fast that it could no longer hold together as a single body. Darwin adopted Poincaré’s line of attack with enthusiasm, and devoted much of the last period of his life to this problem. He was still at work on it at the time of his death, which took place at Cambridge 7 December 1912.
Although the main stream of Darwin’s work was always associated with the evolution of the solar system, yet no small part of his time was spent on quite other problems, many of which were brought to his notice through his membership of various scientific committees. He dealt, as a recognized authority, with a very wide range of subjects, including tidal theory, geodesy, and dynamical meteorology. Of the four large volumes in which his collected works are published [Scientific Papers by Sir George Howard Darwin, 1907-1911], the first is devoted entirely to Oceanic Tides, while the fourth and largest is entitled Periodic Orbits and Miscellaneous Papers. When invited to deliver a course of lectures in Boston, U.S.A., in 1897, he chose as his subject ‘The Tides’. The lectures were subsequently published (1898) in a book which is a masterpiece of semi-popular scientific exposition; it passed through many editions in English, as well as two in German, and has also been translated into Italian, Spanish, and Hungarian.
To the end of his life Darwin’s personality suggested a certain boyish eagerness; he seemed always on the look-out for adventures. He conveyed no suggestion of midnight-oil; his own estimate of his average hours of work was only three a day. That he achieved so much must be ascribed first to a flair for starting each problem in the right way, and secondly to an obstinacy which insisted on probing every problem to the bottom. He lost no time over false starts. His mathematical technique was simple; his method was always that of the direct frontal attack; his skill was of a type which he described just before his death, although with undue self-depreciation, as similar to the skill ‘of a house-breaker who blows in a safe-door with dynamite instead of picking the lock’. Probably his special ability lay in getting his problem set out in perfect order before the dynamiting process began. As a lecturer and speaker he gave a quiet impression of reserve power; his pronouncements being entirely free, as was his whole character, from anything of the nature of display or self-consciousness. His unassuming modesty, no less than his personal charm and eagerness, endeared him to all who met him. He gave his time and energy freely to service on various scientific committees, being especially attracted by such as connected his university or country with the wider world. He acted with conspicuous success as president of the British Association on the occasion of its visit to South Africa in 1905 and was created K.C.B. on his return. In 1909 he presided over the International Geodetic Association, and in 1912, three months before his death, over the International Congress of Mathematicians. His scientific eminence was recognized by numerous honours and by membership of
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