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. These different classes may be rigorously separated. In ancient India they range from the _brahman_ to the _sudra_: in the Europe of the Middle Ages, from the Emperor and the Pope to the feudatory and the vassal, down to the artisan, and an individual cannot pass from one class into another, as his social condition is determined solely by the hazard of birth. Classes may lose their legal character, as happened in Europe and America after the French Revolution, and exceptionally there may be an instance of an individual passing from one class into another, analogously to the endosmose and exosmose of molecules, or, to use the phrase of M. Dumont, by a sort of "social capillarity." But, in any case, these different classes exist as an assured reality and they resist every juridical attempt at leveling as long as the fundamental reason for their differentiation remains. It is Karl Marx who, better than any one else, has proved the truth of this theory by the mass of sociological observations which he has drawn from societies under the most diverse economic conditions. The names (of the classes), the circumstances and phenomena of their hostile contact and conflict may vary with the varying phases of social evolution, but the tragic essence of history always appears in the antagonism between those who hold the monopoly of the means of production--and these are few--and those who have been robbed (expropriated) of them--and these are the great majority. _Warriors_ and _shepherds_ in the primitive societies, as soon as first, family and then individual ownership of land has superseded the primitive collectivism; _patricians_ and _plebeians_--_feudatories_ and _vassals_--_nobles_ and _common people_--_bourgeoisie_ and _proletariat_; these are so many manifestations of one and the same fact--the monopoly of wealth on one side, and productive labor on the other. Now, the great importance of the Marxian law--the struggle between classes--consists principally in the fact that it indicates with great exactness _just what_ is in truth the vital point of the social question and _by what method_ its solution may be reached.