DAE evidence that the “ acquired characters ” upon which they rest are capable of hereditary transmission. The quantitative determination of heredity has been the subject of much patient investigation under the leadership of Francis Galton. The question of isolation as a factor in speciesformation has been greatly discussed, G. J. Romanes proposing, in his hypothesis of “ Physiological Selection,” that the barrier of sterility may arise spontaneously by variation between two sets of individuals as the beginning instead of the climax of specific distinction. Others have fixed their attention upon the variations, which provided the material for natural selection, and have advocated the view that evolution proceeds by immense strides instead of the minute steps in which Darwin and Wallace believed. Others, again, have found significance in the artificial production of “ monstrosities ” or huge modifications during individual development. All through the period a varying proportion of naturalists, probably larger now than at any other time, has followed the founders of the theory, and has sought the motive-cause of evolution in tc the accumulative power of natural selection,” which Darwin, as his first public statement indicates, looked upon “ as by far the most important element in the production of new forms.” They hold, with Darwin and Wallace, that although variation provides the essential material, natural selection, from its accumulative power, is of such paramount importance that it may be said to create new species as truly as a man may be said to make a building out of the material provided by stones of various shapes, a metaphor suggested and elaborated by Darwin, and forming the concluding sentences of The Variation of Animals and Plants under Domestication. This, probably the second in importance of all his works, was published in 1868, and may be looked upon as a complete account of the material of which he had given a very condensed abstract in the first chapter of the Origin, together with the conclusions suggested by it. He finally brought together an immense number of apparently disconnected sets of observations under his “provisional hypothesis of pangenesis,” which assumes that every cell in the body, at every stage of growth and in maturity, is represented in each germ-cell by a gemmule. The germcell is only the meeting-place of gemmules, and the true reproductive power _ lies in the whole of the body-cells which despatch their representatives, hence “pangenesis.” There are reasons for believing that this infinitely complex conception, in which, as his letters show, he had great confidence, was forced upon Darwin in order to explain the hereditary transmission of acquired characters involved in the small proportion of Lamarckian doctrine which he incorporated. If such transmission does not occur, a far simpler hypothesis based on the lines of Weismann’s “ continuity of the germ-plasm ” is sufficient to account for the facts. The Descent of Man, and Selection in Delation to Sex, ■was published in 1871; as the title implies, it really consists of two distinct works. The first, and by far the shorter, was the full justification of his statement in the Origin that “ light would be thrown on the origin of man and his history.” In the second part he brought together a large mass of evidence in support of his hypothesis of sexual selection which he had briefly described in the 1858 essay. This hypothesis explains the development of colours and structures peculiar to one sex and displayed by it in courtship, by the preferences of the other sex. The majority of naturalists probably agree with Darwin in .e ievmg that the explanation is real, but relatively unimportant. It is interesting to note that only in this subject and those treated of in the Variation under Domestication had Darwin exhausted the whole of the material which he a collected. The Expression of the Emotions, published
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in 1872, offered a natural explanation of phenomena which appeared to be a difficulty in the way of the acceptance of evolution. In 1876 Darwin brought out his two previously published geological works on Volcanic Islands and South America as a single volume. The widely read Formation of Vegetable Mould through the Action of Worms appeared in 1881. He also published the following volumes on Botanical subjects. The Fertilization of Orchids appeared in 1862. The subject of cross-fertilization of flowers was in Darwin’s mind, as shown by his notebook in 1837. In 1841 Robert Brown directed his attention to Sprengel’s work (Berlin, 1793), which confirmed his determination to pursue this line of research. Thx Effects of Cross- and Self-Fertilization in the Vegetable Kingdom (1876) contained the direct evidence that the offspring of cross-fertilized individuals are more vigorous, as well as more numerous, than those produced by a selffertilized parent. Different Forms of Flowers on Plants of the Same Species appeared in 1877. It is here shown that the different forms, although each possesses both kinds of sexual organs, is specially adapted to be fertilized by the pollen of another form, and that when artificially fertilized by its own pollen less vigorous offspring, bearing some resemblance to hybrids, are produced. He says, “ no little discovery- of mine ever gave me so much pleasure as the making out the meaning of heterostyled flowers ” (Autobiography').' Climbing Plants was published in 1875, although it had, in large part, been communicated to the Linnean Society, in whose publications much of the material of several of his other works appeared. This inquiry into the nature of the movements of twining plants was suggested to him in a paper by Asa Gray. The Power of Movement m Plants (1880) was produced by him in conjunction with his son Francis. It was an inquiry into the minute power of movement possessed, he believed, by plants generally, out of which the larger movements of climbing plants of many different groups had been evolved. The work included an investigation of other kinds of plant movement due to light, gravity, &c., all of which he regarded as modifications of the one fundamental movement (circummutation), which exists in a highly specialized form in climbing plants. Insectivorous Plants (1875) is principally concerned with the description of experiments on the Sun-dew (Drosera), although other insect-catching plants, such as Dioncea, are also investigated. Charles Darwin’s long life of patient, continuous work, the most fruitful, the most inspiring, in the annals of modern science, came to an end on April 19, 1882. He was buried in Westminster Abbey on April 26. It is of much interest to attempt to set forth some of the main characteristics of the man who did so much for modern science, and in so large a measure moulded the form of modern thought. Although his ill-health prevented Darwin, except on rare occasions, from attending scientific and social meetings, and thus from meeting and knowing the great body of scientific and intellectual workers of his time, probably no man has ever inspired a wider and deeper personal interest and affection. This was in part due to the intimate personal friends who represented him in _ the circles he was unable frequently to enter, but chiefly to the kindly, generous, and courteous nature which was. revealed in his large correspondence and published writings, and especially in his treatment of opponents. In a deeply interesting chapter of the Life and Letters Francis Darwin has given us his reminiscences of his father’s everyday life. Rising early, he took a short walk before breakfasting alone at 7.45, and then at once set to work, “considering the 14 hours between 8.0 and 9.30 one of his best working times.” He then read his letters and listened to reading aloud, returning to work at about S. III.— 49
