  Wright held that three different “classes of causes” are involved in natural selection. The first has to do with the external conditions of the life of a living thing, its relation to other organisms and the non-organic world. Second are physical laws; he mentions specifically principles of mechanics, optics, and acoustics. These are the best known and most basic of all the principles of science. They are the principles by which means come to be fitted to ends, the fulfilling or supplying of the needs of the organism. They are the laws in accordance with which an arm or wing, an eye or ear, can be of use. Third are the causes introduced by Darwin, “the little known phenomena of variation, and their relations to the laws of inheritance” (PD 142). He said there are several divisions within this third class, distinguishing in particular diversities always existing in a population from abnormal or unusual variations. In responding to St. George Mivart's criticism of natural selection, he said that diversities existing normally in a population are the source of evolutionary change more than “unusual and monstrous variations” (PD 144). Wright made this point both to highlight the level at which natural selection operates and to drive home the role of natural selection as an alternative to teleological explanations of the usefulness of adaptations. Variations in inherited characteristics in individuals are not themselves the direct causes of changes in species. Natural selection is a complex general fact of which utility is the organizing principle.