In the same way all the machinery of social evolution has been reduced by Marxian socialism to the law of the _Struggle between Classes_. This theory not only gives us the secret motive-power and the only scientific explanation of the history of mankind; it also furnishes the ideal and rigid standard of discipline for political socialism and thus enables it to avoid all the elastic, vaporous, inconclusive uncertainties of sentimental socialism. The only scientific explanation of the history of animal life is to be found in the grand Darwinian law of the _struggle for existence_; it alone enables us to determine the natural causes of the appearance, development and disappearance of vegetable and animal species from paleontological times down to our own day. In the same way the only explanation of the history of human life is to be found in the grand Marxian law of the _struggle between classes_; thanks to it the annals of primitive, barbarous and civilized humanity cease to be a capricious and superficial kaleidoscopic arrangement of individual episodes in order to become a grand and inevitable drama, determined--whether the actors realize it or not, in its smallest internal details as well as in its catastrophes--by the _economic conditions_, which form the indispensable, physical basis of life and by the _struggle between the classes_ to obtain and keep control of the economic forces, upon which all the others--political, juridical and moral--necessarily depend. I will have occasion to speak more at length--in studying the relations between sociology and socialism--of this grand conception, which is the imperishable glory of Marx and which assures him in sociology the place which Darwin occupies in biology and Spencer in philosophy.[37] For the moment it suffices for me to point out this new point of contact between Socialism and Darwinism. The expression, _Class-Struggle_, so repugnant when first heard or seen (and I confess that it produced this impression on me when I had not yet grasped the scientific import of the Marxian theory), furnishes us, if it be correctly understood, the primary law of human history and, therefore, it alone can give us the certain index of the advent of the new phase of evolution which Socialism foresees and which it strives to hasten. To assert the existence of the class-struggle is equivalent to saying that human society, like all other living organisms, is not a homogeneous whole, the sum of a greater or smaller number of individuals; it is, on the contrary, a living organism which is made up of diverse parts, and their differentiation constantly increases in direct ratio to the degree of social evolution attained.