As we look back upon the past from the vantage ground of modern insight we see that men of the loftiest powers could be blind to intimations now plain and clear. Many a time have designers and inventors paralleled, without knowing it, some structure of nature often seen but never really observed. All the variety and beauty of the Greek orders of architecture failed to include the arch; yet the contour of every architect’s own skull was the while displaying an arched form which could lend to temple and palace new strength as well as grace. The skeleton of the foot reveals in the instep an arch of tarsal and metatarsal bones, with all the springiness which their possessor may confer upon a composite arch of wood or steel. Modern builders, whether wittingly or not, have taken a leaf from the book of nature in rearing their tallest structures with hollow cylinders of steel. What is this but borrowing the form of the reed, the bamboo, a thousand varieties of stalk, one of the strongest shapes in which supporting material can be disposed? Pass a knife across a blade of pipe or moor grass and you will find a hollow cylinder stayed by buttresses numbering nearly a score. More elaborate and even more gainful is the way in which tissue grows in the columns of dead-nettles and bulrushes. The bones in one’s arms and legs resemble the hollow cylinders of which these stalks show instructive variations, so that without going beyond his own frame the designer could long ago have learned a golden lesson. How bone is joined to bone is scarcely less remarkable, as in the braces of the thigh bone as it joins the trunk. As bones move upon each other all shock is prevented by a highly elastic cushion: the springs of vehicles, the buffers of railroad trains, but repeat the cartilages in the joints of their inventors. [Illustration: Section of pipe or moor grass.] [Illustration: Cross-section of bulrush, _Scirpus lacustris_.] In the theodolite and sextant, in the geometric lathe of the bank-note engraver, are ball-and-socket joints allowing motion in any plane. Equally free in their movements are the shoulder and hip joints, while their surfaces are lubricated by a delicate synovial fluid supplied just as it is wanted. When pumps first received valves to direct their flow in one direction, their inventor was no doubt gratified at his skill. In the heart within his own breast, in his veins and arteries, were simple valves engaged in a similar task as they directed the currents of his blood. In pumps such as are common in farm-yards, the action is jerky, the stream flowing and ebbing from moment to moment as the arm rises and falls. The tide of human blood would have the same uneven pulse were it not for the elasticity of its arterial walls. Their elasticity serves to equalize the flow, much as the air does in large chambers on pumps for mines or waterworks.