Here we see that Marxism and Darwinism are not two independent theories, each of which applies to its special domain, without having anything in common with the other. In reality, the same principle underlies both theories. They form one unit. The new course taken by men, the substitution of tools for natural organs, causes this fundamental principle to manifest itself differently in the two domains; that of the animal world to develop according to Darwinian principle, while among mankind the Marxian principle applies. When men freed themselves from the animal world, the development of tools and productive methods, the division of labor and knowledge became the propelling force in social development. It is these that brought about the various systems, such as primitive communism, the peasant system, the beginnings of commodity production, feudalism, and now modern capitalism, and which bring us ever nearer to Socialism. X. CAPITALISM AND SOCIALISM. The particular form that the Darwinian struggle for existence assumes in development is determined by men’s sociability and their use of tools. The struggle for existence, while it is still carried on among members of different groups, nevertheless ceases among members of the same group, and its place is taken by mutual aid and social feeling. In the struggle between groups, technical equipment decides who shall be the victor; this results in the progress of technique. These two circumstances lead to different effects under different systems. Let us see in what manner they work out under capitalism.