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.

Just as a protozoon is almost wholly composed of albuminoid gelatine, while a mammal is composed of tissues widely varying in kind, in the same way a tribe of primitive savages, without a chief, is composed simply of a few families and the aggregation is the result of mere material propinquity, while a civilized society of the historical or contemporaneous period is made up of social classes which differ, the one from the other, either through the physio-psychical constitutions of their component members, or through the whole of their customs and tendencies, and their personal, family or social life.