system to move to particular ends in the interest of maintenance, it can not move to such ends save in the direction of the least resistance. Thus, if a stone thrown at a mark takes the shortest route, having regard to the whole of the influences which act upon it, so a pedestrian goes to his destination by the shortest way which the circumstances permit. A volume of steam finds exit from an overstrained boiler at the weakest point; so by the weakest point an animal escapes from its cage. As a river flows through its channel, determined to that path by the resistance which prevents deviation from it, so the traveler is held to the beaten track by the broken and difficult ground on each side of it. A bullet is diverted by some obstacle suddenly encountered; the root of a plant coils round the stone it meets; the railway engineer usually carries his line round an obstruction instead of through it; the secondary current of an induction coil avoids a journey of many miles by leaping through a flaw in the insulation; a dishonest pupil avoids work at examination by copying the replies of a fellow-student. The light wave makes its way, roughly speaking, spirally through ether; objects of large surface and slow descent, such as certain suitably shaped pieces of paper, descend through the atmosphere in a spiral path; a bubble of air ascends spirally through water; the plant climbs a tree by spiral windings; a horse mounting a steep ascent with a heavy load takes a zigzag or spiriform course; men ascend and descend by spiral stair-ways; water sinks through an orifice spirally, and the descent of a whirlpool is a spiral; boring instruments, such as gimlets, augers, corkscrews, have spiral blades. The hunter seeks particular animals at pools and watercourses which they frequent, as certain medusæ throng to water traversed by a beam of light because the illumination attracts thither small crustacea upon which they feed. Earthworms, in drawing leaves into their holes, seize the leaf at such a point as will permit its passage into the hole with the least amount of resistance; a man carrying a ladder on his shoulder through a crowded thoroughfare carefully regulates his movements so as to avoid collisions. Men escape from an invested city by utilizing the wind; the invested dandelion balloons its seed to a place where it can grow in safety. Certain organisms wear the garb of others in order to increase the ease of their existence; certain men mimic their fellows to the like end of diminishing resistance. As a mother disguises her child's medicine in sugar or sirup, so plants offer their seeds to animals in sweetly flavored fruits. Bees construct their combs in the form that secures the utmost capacity for storage with the smallest expenditure of building material and therefore of energy; so human builders attain in their constructions a maximum of needed effect with the lowest minimum expenditure of material and labor. A general
