THE PRINCIPLE OF ECONOMY IN EVOLUTION. 325 



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 direc- 

 tion of the least stress which we mean when we speak of move- 

 ment 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 encoun- 

 tered 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 determi- 

 nations 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 cos- 

 mical 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 



