The secrets of Nature often play like an iridescence on the surface, and escape the eye of her worshipper because it is stopped with a microscope. There are mysteries all about us as omnipresent as the movement of the air that lifts the smoke and stirs the leaves, which I cannot find that any philosopher has looked into. Often and deeply have I been impressed with this. For example, there is scarcely, in this world, a commoner or a humbler thing than a tail, yet how multifarious is it in aspect, in construction, and in function, a hundred different things and yet one. Some are of feathers and some of hair, and some bare and skinny; some are long and some are short, some stick up and some hang down, some wag for ever and some are still; the uses that they serve cannot be numbered, but one name covers them all. In the course of evolution they came in with the fishes and went out with man. What was their purpose and mission? What place have they filled in the scheme of things? In short, what is the true inwardness of a tail? If we try to commence--as scientific method requires--with a definition, we stumble on a key, at the very threshold, which opens the door. For there is no definition of a tail; it is not, in its nature, anything at all. When an animal's fore-legs are fitted on to its backbone at the proper distance from the hind-legs, if any of the backbone remains over, we call it a tail. But it has no purpose; it is a mere surplus, which a tailor (the pun is unavoidable) would have trimmed off. And, lo! in this very negativeness lies the whole secret of the multifarious positiveness of tails. For the absence of special purpose is the chance of general usefulness. The ear must fulfil its purpose or fail entirely, for it can do nothing else. Eyes, nose and mouth, hands and feet, all have their duties; the tail is the unemployed. And if we allow that life has had any hand in the shaping of its own destiny, then the ingenuity of the devices for turning the useless member to account affords one of the most exhilarating subjects of contemplation in the whole panorama of Nature. The fishes fitted it up at once as a twin-propeller, with results so satisfactory that the whale and the porpoise, coming long after, adopted the invention. And be it noted that these last and their kin are now the only ocean-going mammals in the world. The whole tribe of paddle-steamers, such as seals and walruses and dugongs, are only coasters.