 Multiple bouts of Darwin’s process involving different traits, acting sequentially or in concert, may then explain both how speciation and the evolution of complex adaptations occur through the gradual evolution (change over time) of natural populations. Darwin aimed to convince his audience that even such structures as the vertebrate eye, which at first seem explicable only as the product of design, could instead be explained by incremental, directional evolution, a complex but still naturalistic process (Darwin 1859: ch. 6). What is initially a light sensitive patch may be transformed into an eye by means of a great many bouts of selection that gradually improve and enhance its sensitivity. Showing that something is explicable is importantly different from explaining it (Lennox 1991); still, a theory must be an explanatory sort of theory for it to accomplish either task. After Darwin, the appearance of novel species in the geological record and the existence of designed-appearing adaptations cannot be used as grounds for invoking supernatural causes as a matter of last explanatory resort.