His greatest book next to the _Origin_—containing an immense quantity of original notes and observations and valuable information from all kinds of breeders and fanciers—is the _Variation of Animals and Plants under Domestication_ (1868). The facts recorded are discussed in the light of the great theory, and honest, fair-minded consideration is given to those which present difficulties as well as to those which clearly favour it. In 1871 came the _Descent of Man_, followed in 1872 by the _Expression of the Emotions in Men and Animals_—in which, again, it was shown that the facts as to the likeness between man and apes can be explained on the theory that natural selection and survival of favourable variations have been at work, and that the facts are hopelessly without meaning or explanation on any other hypothesis. His last published book was on _The Formation of Vegetable Mould through the Action of Worms_, in which he not only showed what an important part earthworms play in burying stones and rocks, and in fitting the ground for the growth of plants, but recorded some discoveries as to the senses of worms and as to their treatment of leaves by a digestive fluid exuded from the mouth so as to soften a leaf before swallowing it. Every one of Darwin’s books abounds with new facts and new points of view disclosed by the application to first one thing and then another of his vivifying discovery-causing theory of natural selection. The subsidiary theory of the selection of brilliantly coloured males by females in pairing, as a cause of the brilliant colours and patterns of many birds and insects, is developed in his _Descent of Man_. It led him to many important discoveries and observations as to the colouring and ornamentation of animals, and when considered, together with Wallace’s and Bates’s theory of mimicry and of the warning and protective colourings of insects, goes far to explain all the specific colouring of animals and plants as due to natural selection and survival. A theory which has produced such prodigious results in the way of “explaining” all forms, colours, habits, and occurrences of living things—as has that of Charles Darwin—simply holds the field against all comers. When Lamarck’s theory has been shown to be consistent with the most elementary facts as to heredity, and further to afford a rational explanation of any group of biological facts, it will be time to consider how far it may be entertained in conjunction with Darwin’s theory—but not until then. V DARWIN’S THEORY UNSHAKEN