here within the limits of a single species we have a degree of sterility strictly comparable to what obtains in the crossing of distinct species. Thus our knowledge of heterostyled plants is of importance as bearing on one of the most difficult points in the statement of the case for evolution, the sterility of inter- specific crosses, and of hvbrids. The papers on heterostyled plants (the first of which was published in 1862), supplemented with a number of facts and discussions of a cognate kind, formed the basis of the book on 'Different Form6 of Flowers,' which appeared in 1877. The work on climbing plants had a somewhat similar history, inasmuch as it was first published (1864) by the Linnean Society, and afterwards republished (1875) as a separate book. The subject was suggested to him by a paper of Dr. Asa Gray's published in 1858, and he was the more attracted to it because he was not satisfied with the explanation of the mechanism of twining taught by Henslow at Cambridge. The problem had been at- tacked by two German physiologists before Darwin wrote, but he was ignorant of this fact when he began to observe climbing plants, and his work has a value quite independent of theirs. It was a subject he enjoyed greatly, for, as he has said, 'some of the adaptations displayed by climbing plants are as beautiful as those of orchids for ensuring cross-fertilisation.' This book did not lead him at once to any wide theoretical conclusions, but it was the starting-point of his last book, the 'Power of Movement in Plants.' In working at climbing plants he had to study the revolving movement of growing shoots, and when he found that these movements, as exhibited by climbing plants, are not confined to any one order of plants, but are found throughout the vegetable kingdom, he was led to speculate on the existence of a fundamental movement which might serve as a basis for the evolution of the complex and striking movements of climbing plants. This movement he found in 'circumnutation,' and the study of circumnutation forms the subject-matter of the 'Power of Movements.' This book required an immense amount of patient work, much of which was of a kind new and difficult to him. It led him to believe that the movements of the growing parts of plants, such as the curvatures which occur in response to the stimulus of light, gravitation, &c., are all modifications of the fundamental element of circumnutation. This conclusion has not been at all universally received by physiologists, and may be said to be still sub judice. But, whether or not subsequent researches sustain his general conclusion, no one, as Mr. Dyer has remarked ('Charles Darwin,' Nature Series), 'can doubt the importance of what Mr. Darwin has done in showing that, for the future, the phenomena of plant movement can, and indeed must, be studied from a single point of view.'
His book on 'Insectivorous Plants,' published in 1875, was the result of the completion and elaboration of observations made many years previously, during a holiday spent in Sussex. Two species of Drosera were abundant at Hartfield, where he was staying in 1860, and he noticed the numerous insects caught by the leaves. The movement of the tentacles was soon seen, and on comparing the behavour of leaves placed in nitrogenous with those in non-nitrogenous fluids, it became evident that 'here was a fine new field for investigation.' The subject was occasionally taken up in subsequent years (often in a spirit of incredulity at his own results), and during the summers of 1872 and 1873, and the greater part of 1874, he worked steadily at it.
Darwin always enjoyed experimental work far more than writing, and the pleasure of following out the brilliant discoveries which he made in the natural history of insectivorous plants was a relief and rest to him from the drier labour of preparing a second edition of the 'Descent of Man.'
Darwin's last publications were two papers of no great importance, read before the Linnean Society in the autumn of 1881. They dealt with a kind of coagulation or 'aggregation' produced in certain leaves and roots by the action of ammonia. It was thus a piece of work directly connected with 'Insectivorous Plants,' where for the first time the curious process of aggregation, as seen in the tentacles of Drosera, was described. Darwin's views as to the nature of aggregation have now been shown to be erroneous ; nevertheless the investigation possesses a permanent interest and importance as a contribution to the physiology of the cell.
Personal Characteristics. — In figure Darwin was thin and tall, being about six feet in height, though from a slight habitual stoop he scarcely looked so tall. His frame was naturally strong, and fitted for activity, but he had a certain clumsiness of movement, shown, for instance, in his inability to use his hands in drawing. As a young man he had much endurance, and during an expedition from the Beagle he was one of the few who were able to struggle on in search of water when all were suffering from thirst and exhaustion.
His face was ruddy, his eyes blue-grey under deep overhanging brows and bushy eyebrows. His high forehead waa much
