His stimuli are also limited. This small machine is kept going by its own supply of nervous energy, replenished by food, sleep, air, and water. It will run so long, and then must rest and be “fired up.” Special excitants of fear, pain, or unusual hunger may temporarily accelerate his activity, but he has then to rest the longer. His executive capacity is thus limited, second, by his small nervous energy and narrow range of stimulus. It is further confined, thirdly, by the narrow circle of his instincts, desires, or ideas, if he has them. The governing impulse is simple race-preservation, mingled with the self-preserving instincts; egoism and familism cover his range of interests. Hope, fear, desire,—all are for self or family. So we find in the individual animal, his efficiency is limited by (_a_) his personal mechanism, (_b_) his personal nerve force, and (_c_) his personal interests. For such an agent work—continuous expression of energy—would indeed be difficult. But now examine the position of the human being. Man’s tools do not grow on him. He has been able to evolve improved tools without sacrificing a thousand slow generations to breed them. He adds to his executive ability, (_a_) the power of numbers, and of the “relay race” (wild dogs have this), (_b_) the power of division of labour, (ants and bees have this), (_c_) the tool, detachable and exchangeable. In this comes at once an enormous saving of energy. Where the mole has to spend not only his immediate strength in digging, but his whole racial tendency in being modified to digging, the man with a spade can do far more work in proportion to his strength, and still be able to do other things. The executive efficiency of the man is multiplied, first, by association, again by division of labour, and again by the tool. The tool being not a personal adjunct like the claw, but a separate thing, usable by many, the efficiency is again increased by the exchange of tools. It is multiplied, fourth, by the development of the tool into the machine, and fifth, by the application to the machine of extra-personal power, of the forces of nature direct. Thus where one man alone as a separate naked animal could accomplish something equal to, say 5: as a member of society his efficiency is squared by association = 25; cubed by the division of labour = 125; raised to the fourth power by the tool = 625; to the fifth, by the machine = 3125; and to the sixth, by the use of natural forces = 15,625.