One of the most marvellous aspects of Darwin's work was the way in which he seemed always to throw a new light upon every department of inquiry into which the course of his researches led him to look. The specialists who, in their own narrow domains, had been toiling for years, patiently gathering facts and timidly drawing inferences from them, were astonished to find that one who, to their eyes, was a kind of outsider, could point out to them the plain meaning of things which, though entirely familiar to them, they had never adequately understood. The central idea of the _Origin of Species_ is an example of this in the biological sciences. The chapter on the imperfection of the geological record is another in the domain of geology. After the publication of the _Origin_, Darwin gave to the world during a succession of years a series of volumes, in which some of his observations and conclusions were worked out in fuller detail. His books on the fertilisation of orchids, on the movements and habits of climbing plants, on the variation of animals and plants under domestication, on the effects of cross and self-fertilisation in the vegetable kingdom, on the different forms of flowers in plants of the same species, were mainly based on his own quiet work in the greenhouse and garden at Down. His volumes on the descent of man, and on the expression of the emotions in man and animals, completed his contributions to the biological argument. His last volume, published the year before his death, treated of the formation of vegetable mould, and the habits of earth-worms, and the preparation of it enabled him to renew some of the observations which had interested him in his younger days, and to revive some of the geological enthusiasm which so marked the earlier years of his life.