Darwin's independent criterion of fitness is, indeed, "improved design," but not "improved" in the cosmic sense that contemporary Britain favored. To Darwin, improved meant only "better designed for immediate, local environment." Local environments change consistently: they get colder or hotter, wetter or drier, more grassy or more forested. Evolution by natural selection is no more than a tracking of these changing environments by differential preservation of organisms better designed to live in them: hair on a mammoth is not progressive in any cosmic sense. Natural selection can produce a trend that tempts us to think of more general progress—increase in brain size does not characterize the evolution of group after group of mammals. But big brains have their uses in local environments; they do not marked intrinsic trends to higher states. And Darwin delighted in showing that local adaptation opted produces "degeneration" in design—anatomical simplification in parasites, for example.
