Darwin discovered that the presence of the same species of plants and of some few animals on distant mountain summits and in the Arctic region is due to the former extension of ice between these situations during the last glacial period. He was, before everything else and by necessity for the examination of his theory, a geologist, and wrote many valuable geological memoirs. The history of the origin of the species of living things consists largely in tracing them to extinct creatures, and in showing what were the possible migrations and what the conditions of land and water, temperature and vegetation, in past periods, and in regard to given areas of the globe. The book on the _Fertilisation of Orchids_ was the first published by Darwin after the _Origin of Species_. In it he showed how the marvellous shapes and colours and mechanisms of the flowers of orchids are adapted to ensure cross-fertilisation by insects, and how they can be explained as originating by the natural selection of variations—if the value of cross-fertilisation is once recognised. The explanation of the reason for the existence of two kinds of primrose flowers—the short-styled and the long-styled—clearly arrived at by him as being a mechanism to secure cross-fertilisation, delighted him in 1862, and led him to discover the same sort of modification in other flowers. Then, in 1864, he published his researches on _Climbing Plants_, and later a book on the _Movements of Plants_, in which he discovered the mechanism and the wonderful variety of movements of plants, and showed their value to the plant, and consequent origin, by natural selection. He especially loved to discover evidence that plants can do many things which had been thought to be only within the powers of the other section of living things—the animals; and finding during one summer holiday that the beautiful little sun-dew moves its red-knobbed tentacles so as to entrap minute insects, he discovered the whole history of _Insectivorous Plants_, and showed that there are many plants of various groups which catch insects and digest them in a sort of stomach, as an animal might do. Thus the water-holding pitchers of the pitcher-plants of tropical forests were explained as being food-catchers and digesters of great value to the nutrition of the plant, and their gradual formation by variation and natural selection rendered comprehensible.