The life of a theory may be long and dramatic: we may speak of its conception, fetal existence, birth, and infantile development; of triumph and defeat; of alternating phases of rejection and acceptance, domination and humiliation, stagnation and senility; and of astonishing rejuvenation and revival. Dark periods of neglect may alternate with stages of excessive popularity. (Seen in this perspective, it is not astonishing that the history of scientific theories now entices so many brilliant students of history!)