 Wright was in advance of his contemporaries in his understanding of Darwin's change in organisms and species, in part because he applied the foregoing interpretation of science to Darwin's theory. Wright highlighted the overall structure of the theory of evolution, which he believed illustrated the principle of utility. He also characterized evolutionary change in terms of different levels of causative and explanatory principles. Natural selection is a descriptive principle that unifies these other principles in a comprehensive account. It is a template, a form of explanation, by which an investigator may be guided in finding how more basic explanatory principles — the principles of chemistry and the laws of inheritance, for instance — issue in features of living things observable by direct perception.