The socialists, in fact, think that the laws of life are the following, and that they are concurrent and inseparable: _the struggle for existence_ and _solidarity in the struggle against natural forces_. If the first law is in spirit individualist, the second is essentially socialistic. Now, not to repeat what I have written elsewhere, it is sufficient here for me to establish this positive fact that all human evolution is effected through the constantly increasing predominance of the law of solidarity over the law of the struggle for existence. The forms of the struggle are transformed and grow milder, as I showed as long ago as 1883, and M. Garofalo accepts this way of looking at the matter when he recognizes that the muscular struggle is ever tending to become an intellectual struggle. But he has in view only the formal evolution; he wholly disregards the progressive decrease in the importance of the struggling function under the action of the other parallel law of solidarity in the struggle. Here comes in that constant principle in sociology, that the social forms and forces co-exist always, but that their relative importance changes from epoch to epoch and from place to place. Just as in the individual egoism and altruism co-exist and will co-exist always--for egoism is the basis of personal existence--but with a continuous and progressive restriction and transformation of egoism, corresponding to the expansion of altruism, in passing from the fierce egoism of savage humanity to the less brutal egoism of the present epoch, and finally to the more fraternal egoism of the coming society; in the same way in the social organism, for example, the military type and the industrial type always co-exist, but with a progressively increasing predominance of the latter over the former. The same truth applies to the different forms of the family, and also to many other institutions, of which Spencerian sociology had given only the _descriptive_ evolution and of which the Marxian theory of economic determinism has given the _genetic_ evolution, by explaining that the religious and juridical customs and institutions, the social types, the forms of the family, etc., are only the reflex of the economic structure which differs in varying localities (on islands or continents, according to the abundance or scarcity of food) and also varies from epoch to epoch. And--to complete the Marxian theory--this economic structure is, in the case of each social group, the resultant of its race energies developing themselves in such or such a physical environment, at I have said elsewhere.