take place in the presence of or against resistance, a body which moves in the line of the greatest stress necessarily moves in the direction of the least stress, and it is this movement in the direction of the least stress which we mean when we speak of movement in the direction of the least resistance.
We have next to note what is meant by the greatest stress. This is not necessarily a stress applied at a single moment in time or at a single point in space. The movement of a billiard ball, for example, may be determined for part of its course wholly by the blow given with the cue, but the cushions soon come into action, and thus the total course of the ball is decided, not solely by the cue, but by the cue and all subsequent stresses of the cushions and balls that happen to be struck. In like manner, the initial impulse is given to the cannon ball by the exploding gunpowder, yet this initial stress is immediately complicated with gravitative action; and when we say that such a ball moves in the line of the greatest stress, we mean not simply the direction originally given by the cannon, but the whole direction as determined by cannon, gravity, and atmosphere. The greatest stress determining the direction of movement, then, is a stress made up, not only of the initial stress, but also of all subsequent determinations encountered as resistances by the moving body in its course; and when we say that a body moves in the direction of the least resistance, we mean that its total movement is determined by the total of greatest stresses. It is true that a distinction may be made between the original impulse given to a body and the subsequent stress or stresses entailed upon it by its own movement, and due to contact with other bodies at rest or in motion. It is an active stress, for example, which gives the initial impulse to the billiard ball; it is a reactive stress by which the cushion deflects the ball from its original course. But this distinction is little more than formal; the whole of the stresses determining movement, however easy it may be to analyze them into parts, must be regarded in their totality; for if we have to account for movements that take place in time in their totality, we must consider the determinations to those movements in their totality.
The law of least resistance, as we may briefly call it, finds exemplification alike in the realm of life and in the world of inanimate things. Not only are all movements of masses and their parts—from the descent of a bowlder down the hillside to the revolutions of planets in their orbits; from the activities of gas molecules in a chemist's laboratory to the movements of cosmical aggregation out of which suns arise—due to a differential stress producing motion in the presence of resistance to that motion: the law is valid also for the activities of animals, since, if the molecular forces embodied in an organic system impel that
