Then he climbed down from his abode in trees, and began the conquest of the earth. The club made man a traveler. His forays with that weapon taught him to walk and fight upon his hind legs, and gave him his erect carriage. But he had to travel a long and thorny pathway indeed, armed only with a club, before he invented the stone hatchet and spear of sharpened flint or bone. It was a far-flung span across the gulf of time from the tree-home to the cave in the hill, his new abiding-place. The bow and arrow, which enabled him to kill at long range, were his next weapon, and were the greatest invention of all time. The protection of the heart with the left arm and shield, with the right arm free to wield the sword or hurl the javelin, made man right-handed. Armed with the bow and arrow, spear and shield, man was equipped still better for travel; and ever since travel has been widening out the sky and broadening man's mental horizon. The fighting spirit widened the acquaintance of different peoples, and the terrible menace of some savage common enemy forced different tribes to unite and build up nations. Union against danger is the best instructor of self-government, and the best guarantee of internal good behavior. It is generally recognized that man is a product of his environment; that he is in body and mind the sum of his own and ancestral experiences; that he is omnivorous; that he drinks water and breathes air; and yet, many persons fail to recognize the inevitable concomitant conclusion that he is also of necessity a warring animal, and that the formative influences of the fierce struggle for existence have made him what he is. His life is a series of reactions to environing stimuli; and he is actuated and shaped by those stimuli, and just as those stimuli have been necessary to his growth, so they are still necessary to his continued growth, and even to his very existence. In other words, the formative influences that have made and sustained man are still necessary to his maintenance. The character of the strife may be changed, and is already largely changed, from war to business. But the intensity of the struggle cannot be alleviated one whit, because it is impossible, in the nature of things, to maintain man's strength of character in any other way. He could live a little longer without strife than without food or air or water, but the absence of strife would be as fatal to him in the end as would be the absence of food, air, or water. The struggle for existence has always been a business proposition with man, and business today is a struggle for existence as intense and merciless as the struggle in war.