I remember well the time when the thought of the eye made me cold all over, but I have got over this stage of the complaint, and now small trifling particulars of structure often make me very uncomfortable. I hope that my talk this afternoon does not make someone ill. Their complexity suggests a very rich evolutionary history.
Feathers are most important in thermoregulation, locomotion, and communication.
However, it seems unlikely that feathers spontaneously began as such a dynamic structure, instead, over time, various selection forces allowed new features to be incorporated into the whole. The first suggests that feathers evolved directly for flight through a series of stages. The other proposes that feathers were originally used like hair for thermoregulation. I would hazard a guess that most everyone in this room subscribes to one of these two hypotheses. Some of you might know of it as the display hypothesis.