In a great city each branch post office is connected solely with headquarters, to which it sends its letters, papers, and parcels, receiving in return its batches for local distribution. For each branch office to communicate with every other would be so costly and cumbrous a plan as to be quite impracticable. Our postal method is adopted in every telephonic service; Z communicating with D or M only after he has had his line joined to the central switchboard which connects with every telephone in the whole system. All this was prophesied in the remote ancestry of both postmasters and electricians as their nerves took the paths of what is in effect a complete telegraphic circuit, with separate up and down lines and a central exchange in the brain,--that prototype of all other means of co-ordination. Fibrils of the Ear and Eye. Pianos, organs, and other musical instruments yield their notes by the vibration of strings, pipes, or reeds of definite size and form. Across the larynx, the box-like organ of the throat, the vocal cords vibrate in an identical way. When we sing a note into an open piano, the string capable of giving out that note at once responds. Helmholtz believed that in the ear the delicate, graduated structures, known as the rods of Corti, vibrate in the same way when sound-waves reach them, giving rise to auditory impressions. Analogous in operation are the fibrils of the eye which respond to light-waves of various length and intensities. The human eye has muscles which modify its globularity, rendering its lenses more or less convex. A cat has a higher degree of this kind of ability, so that it can dilate its pupil so much as to see clearly in a feeble light. A man who remains in a darkened room so rests his nerves of vision that in four or five hours he can readily discern what would be unseen were he newly brought into the darkness. The Electric Eel. Not only in the frame of man, but in the bodies of the lower animals, are suggestions which ingenuity might well have acted upon in the past, or worthily pursue in the future. The science of electricity was born only with the nineteenth century because the gymnotus, or electric eel, had not been understandingly dissected. Its tissues disclose the very arrangement adopted by Volta in his first crude battery, namely, layers of susceptible material surrounded by slightly acid moisture. The characteristics of this eel have their homologies in the human body; in the muscles which bend the fore-arm, for example, are nearly a million delicate fibrils comparable in structure with the columnar organs of the gymnotus. These fibrils are so easily excited by electricity as to denote an essential similarity of build. Both the columnar layers of the eel and the fibrils of human muscle are affected in the same way by strychnine and by an allied substance, curare. A Beaver Tooth and the Self-Sharpening Plow.