 Further evidence that Darwin doubted a process distinction between species and varieties is found in Chapter 4 of the Origin of Species, titled “Natural Selection.” Darwin proposes two principles, which he calls The Principle of Character Divergence and The Principle of Extinction. Together these principles explain the origin of new taxa and morphological gaps among taxa. The Principle of Character Divergence has a familiar Darwinian starting point. Suppose that a geographic region contains several closely related groups of organisms. Within one of those groups, some organisms are selected because they have a trait that gives them an adaptive advantage. Divergent selection occurs in future generations when organisms with even better adapted forms of that trait are selected, eventually causing pronounced morphological gaps between that group of organisms and its parent and sister groups (Darwin 1859 [1964], 112ff.). Darwin illustrates this process with a number of examples. Consider his example of a pigeon fancier (1859 [1964],112). A pigeon fancier is struck by the slightly longer beak of some birds. She then selects birds with slightly longer beaks in that generation, and continues to do so in subsequent generations until there is a pronounced morphological gap between the selected group and the original stock. Darwin argues that the process of divergent selection causes the origin of new taxa and is the source of branching on the tree of life.