This, indeed, is the only encouragement that I am able to perceive when I watch in history the periodical strife between inveterate conventions and the mother-ideas of genius. In the case of bridges, for example, the first mother-ideas were those that enabled a primitive craftsman here and there to copy with success the least difficult of Nature’s models. What this man achieved was repeated by his tools, the ordinary men of his tribe; then other tribes got wind of the discovery and began to make similar bridges, until at last several conventions were formed, and they became widespread and stereotyped. When a convention was very simple and also effective for a given purpose, no one wished to see it developed, so it entered that domain of infertile mimicry where stone tools and weapons remained unpolished for years to be reckoned by scores of thousands. If experience had shown that chipped flint in a rough state would neither cut wood nor break human skulls, then at an early date polishing would have been found out by a savage of genius who yearned to prove that his invention could be made useful; but rough-hewn stones were rudely efficient, so mankind settled itself in a routine and plodded on and on automatically. And thus it was also in the case of many primitive bridges which became so firmly fixed in conventions that now they seem to be contemporary with nearly all the ages of human strife. Not in any other way can we explain their present use by many Europeans, as well as by the natives of Asia, and Africa, and America (p. 145). On the other hand, when a primeval bridge did not serve its purpose efficiently, when it was useless in tribal wars and dangerous in rainy seasons, then a mother-idea paid it a visit from time to time, as we shall see in the next chapter.