‘Natural selection and the origin of species’ shows that natural selection is the mechanism of evolutionary change, which is not readily observable, but is inferred by argument from other kinds of observation. The argument is constructed from three apparently independent generalisations about the properties of organisms. The first generalisation is that individual members of any species vary from one another in manifold structural and behavioural characteristics. The second generalisation is that individual variation is to some degree hereditary — transmitted from generation to generation. The final generalisation is the Malthusian principle that organisms multiply at a rate that exceeds the capacity of the environment to carry them, with the inevitable consequence that many must die.