Later, Darwin wrote on the fertilisation of plants, in order to demonstrate the importance of cross-fertilisation; on the forms of flowers; and on the movements of plants,--the last a kind of extension and generalisation of the book on climbing plants, endeavouring to co-ordinate all the movements of plants as variations of an inherent tendency of the parts to a revolving motion. The theory has not been accepted by botanists. Last of all, in 1881, appeared the monograph on _The Formation of Vegetable Mould through the Action of Worms_. This book is just the expansion and completion of a paper read by Darwin to a scientific society as far back as 1837. All that time the subject dwelt in his mind; and when at last leisure permitted, he developed it into what is perhaps the most purely delightful of all his books. In greatness it does not come into competition with some of them at all; but the familiarity of the phenomena, the care with which they are examined, the skill of the arrangement and the charm of finding meaning in what had been so meaningless, have made the volume one of the most widely read of all Darwin's works. That which distinguishes Darwin from other naturalists is the combination of extraordinary speculative power with great knowledge of detail and unlimited patience. These qualities have been combined in others as well, but never, within the field of natural history, in the same degree. More commonly they are found separate. The ordinary type of naturalist is the man who knows an immense number of facts about plants and animals, and who rests content with that knowledge. He may be master of everything about the great subject of scarabees, but it scarcely occurs to him to _explain_ the scarabees themselves, still less to use them in explaining other creatures. On the other hand, the opposite type, the type which speculates only without first laying the foundation of fact, is likewise common enough. How ineffectual this is may be seen from the history of earlier speculations on evolution. The _Vestiges of Creation_ and the theory of Lamarck are superseded, not so much because of deficiency in speculative power, as because the theories are not sufficiently buttressed by facts. Even though Darwin's own theory should ultimately be, in one sense, as dead as that of Chambers, it will always remain one of the landmarks of thought.