There is an unfortunate Nemesis attaching to great discoveries and generalizations, which has not been sufficiently noticed. When men have been floundering about for years in a quagmire of confusion and difficulty, and some brilliant pen points out a clear waj' by which the tangle may be threaded,—a way which is so ingenious and simple that it at once seizes upon the current scientific thought, and compels the adhesion of everybody,—it inevitably produces a period of stagnation and for a while dwarfs inquiry. What is so palpably true and simple as the explanation of a very perplexing difficulty is taken to explain all the difficulty, and, for a while at all events, men's energies are devoted to bringing every apparently aberrant and stubborn fact within the new law or the new process; and every effect, however remote, is traced by a direct way or by a zigzag to the prime cause which has proved so fruitful. It is only after an interval, when men have digested and incorporated the new theory into their daily creed, and it no longer overawes them by its freshness, that they are in a mood to ask soberly whether, after all, too much has not been demanded from one cause,—whether, after all, the intricate and far-fetched explanations, which are necessary to connect that cause with effects with which we are familiar, is not illegitimate,—and whether we must not supplement it by some other cause, which has modified and altered its work, if we are to solve the whole riddle.