Nothing is more interesting than to trace, through many years and almost endless wanderings and changes, the fortunes of an idea or habit of thought. The subject is a much-neglected one, even in these days of sweeping and minute investigation, because the inherent difficulties are so great, and the necessary data so multifarious, confused, and sometimes contradictory, that absolute proof and smooth presentation seem well-nigh impossible. Yet the ideas, the opinions, even the prejudices of men, impalpable and indefinite as they are, have at times a wonderful vitality and force and are not without meaning and importance when looked at with considerate eyes. The conditions under which they have been developed may change, or pass utterly away, while they, mere shadowy creations of the mind, will endure for generations. Long after the world to which it belonged has vanished, a habit of thought will live on, indelibly imprinted upon a race or nation, like the footprint of some extinct beast or bird upon a piece of stone. The solemn bigotry of the Spaniard is the fossil trace of the fierce struggle of eight hundred years with the Moors. The theory of the Lord's day peculiar to the English race all over the world is the deeply branded sign of the brief reign of Puritanism. A certain fashion of thought prevailed half a century ago; another is popular to-day. There is a resemblance between the two, the existence of both is recognized, and both, without much consideration, are set down as sporadic and independent, which is by no means a safe conclusion. We have all heard of those rivers which are suddenly lost to sight in the bowels of the earth, and, coming as suddenly again to the surface, flow onward to the sea as before. Or the wandering stream may turn aside into fresh fields, and, with new shapes and colors, seem to have no connection with the waters of its source or with those which finally mingle with the ocean. Yet, despite the disappearances and the changes, it is always the same river. It is exactly so with some kinds of ideas and modes of thought,--those that are wholly distinct from the countless host of opinions which perish utterly, and are forgotten in a few years, or which are still oftener the creatures of a day, or an hour, and die by myriads, like the short-lived insects whose course is run between sunrise and sunset.