3. A savage of genius, perhaps as early as the Tertiary period, cut down a tree in order that it might span a dangerous creek or an abyss in the mountains. Intelligently, with the aid of a flint axe, he copied the work done by many a gale of wind; and in this act of simple mimicry he discovered the first principles of secure bridge-making. The footway was strong, and branches from the tree-trunk gave support to clutching hands. Any bough that blocked up the footway was topped off. Even to-day we find in country woods a good many rustic bridges hewn from tree-trunks, and guarded at the sides by hand rails of dressed branches. Their footways are no wider than the planed surface of a well-grown tree. 4. Another savage of genius, thousands of years later, maybe, took a hint from a troublesome inconvenience which from the first had been present in tree-bridges. The footway being too narrow, he put two or three trees side by side, so that two or three warriors might cross it abreast, instead of weakening their attack by an advance in single file. But this improvement suggested other changes of much greater value both to war and to social life. However carefully the trees were laid side by side, their rounded surfaces left a valley between them; and gaps were formed by curved trunks and by gnarled excrescences. So the widened footway had drawbacks of its own. Often, on a rainy day, naked feet would slip, for the trees were polished by long use; and many a slip would either break or strain an ankle. Yet the wit of mankind would bear these troubles with a grumbling patience; thousands of years may have passed by unprofitably; but sooner or later a man of genius would perceive that every defect in a bridge suggested an improvement. The valley between the tree-trunks could be filled in with soil and pebbles and turf; a round foothold polished by long use and slippery after rain, could be flattened and roughened; and where the trees diverged from each other, making traps for the unwary, invention could be busy for a long time. Why put the trees close together? If they were separated by half a stride, then covered transversely with brushwood and turf, a much better bridge would be made without much effort. Again, suppose the long beams were thin saplings that shook too much underfoot, particularly when a tribe of shouting warriors ran across them in a hot attack. To steady such a bridge with props would be a great convenience, and timber props would serve as conveniently as boulders and piled stones. A criss-cross of logs made an excellent pier,[44] for example, and forked boughs, which entered into several phases of primitive handicraft, made good piles.[45] We know not when these quite simple improvements gave some dignity to manual work, but their inception needed only a little mother-wit. Some Quarternary men ripened a great deal more in their arts, as painters and sculptors and engravers.