But even if we grant that science is our main hope, there remains a choice of methods. On the one hand, there is the way of material progress, physical discovery and feverish haste to apply every new fact to armament; on the other, that of biological research, social enlightenment, and ever-increasing human understanding and sympathy. Firm believers in each of these possible approaches, through science, to international peace, are at hand. The one group argues that nations, like individuals, must be controlled in all supreme crises by fear; the other contends that civilization has developed in enlightened human sympathy a higher, a more worthy, and a safer control of behavior. As a biologist and a believer in the brotherhood of man, I wish to present the merits of sympathy, as contrasted with fear, and to plead for larger attention to the biological approach to the control of international relations. For I am convinced that the greatest lesson of the present stupendous world-conflict is the need of thorough knowledge of the laws of individual and social human behavior. Surely this war clearly indicates that the study of instinct, and the use of our knowledge for the control of human relations, is incalculably more important for the welfare of mankind than is the discovery of new and ever more powerful explosives or the building of increasingly terrible engines of destruction. During the last half-century the physical sciences, technologies, arts and industries, have made marvelous advances. At enormous cost of labor and material resources there have been discovered and perfected means of destroying life and property at once so effective and so terrible to contemplate that preparedness for war seemed a safe guarantee of peace. But who is there now to insist, against the evidence of blood-drenched Europe, that material progress, physical discovery, and armament based thereupon, assure international friendship?