  Early in the Introduction to On the Origin of Species, Darwin observes that the conclusion that each species had descended from others “even if well founded, would be unsatisfactory, until it could be shown how the innumerable species inhabiting this world have been modified so as to acquire that perfection of structure and co-adaptation which most justly excites our admiration” (Darwin 1859, 3). One might say this was the central promise of Darwinism—to account for both phylogenic continuity and adaptive differentiation by means of the same principles; or as Darwin puts it, to integrate in one theory the supposed opposition between Unity of Type and Conditions of Existence.