fit or specific role, followed by a primary exaptation (a new role is derived using the existing feature but may not be perfect for it), which in turn leads to the development of a secondary adaptation (the feature is improved by natural selection for better performance), promoting further development of an exaptation, and so forth. Once again, feathers are an important example, in that they may have first been adapted for thermoregulation and with time became useful for catching insects, and therefore served as a new feature for another benefit. For instance, large contour feathers with specific arrangements arose as an adaptation for catching insects more successfully, which eventually led to flight, since the larger feathers served better for that purpose. Implications Evolution of complex traits One of the challenges to Darwin's theory of evolution was explaining how complex structures could evolve gradually, given that their incipient forms may have been inadequate to serve any function. As George Jackson Mivart (a critic of Darwin) pointed out, 5 percent of a bird wing would not be functional. The incipient form of complex traits would not have survived long enough to evolve to a useful form. As Darwin elaborated in the last edition of The Origin of Species, many complex traits evolved from earlier traits that had served different functions. By trapping air, primitive wings would have enabled birds to efficiently regulate their temperature, in part, by lifting up their feathers when too warm. Individual animals with more of this functionality would more successfully survive and reproduce, resulting in the proliferation and intensification of the trait. Eventually, feathers became sufficiently large to enable some individuals to glide. These individuals would in turn more successfully survive and reproduce, resulting in the spread of this trait because it served a second and still more beneficial function: that of locomotion. Hence, the evolution of bird wings can be explained by a shifting in function from the regulation of temperature to flight. Jury-rigged design Darwin explained how the traits of living organisms are well-designed for their environment, but he also recognized that many traits are imperfectly designed. They appear to have been made from available material, that is, jury-rigged. Understanding exaptations may suggest hypotheses regarding subtleties in the adaptation. For instance, that feathers evolved initially for thermal regulation may help to explain some of their features unrelated to flight. However, this is readily explained by the