Baudelaire compared the great names in art to lighthouses posted along the track of historic time. The simile, as he used it, seizes the imagination and represents a great truth, but it allows of an interpretation which the limits of a sonnet form forbade him to develop. He takes the lights of his beacons as much for granted as the sailor does the lights of real lighthouses. But the lighthouses of art do not burn with so fixed and unvarying a lustre. The light they give is always changing insensibly with each generation, now brighter, now dimmer, and often enough growing bright once more. But we sometimes forget that the lights have to be tended or they grow faint and may expire altogether. For them to burn brightly, they must be fed by the devotion of some few spirits in each generation. If that fails for a long period they go out and become one of those dead, ineffectual names which still linger on, obstructions rather than aids to the historical voyager.