(1) In order to ascertain the causes of the solidarity between the different habits of one and the same society, it is necessary to look beyond the abstract and conventional form which the facts assume in language (dogma, rule, rite, institution), and attend to the real concrete centres, which are always thinking and acting men. Here only are found together the different species of activity which language separates by abstraction. Their solidarity is to be sought for in some dominating feature in the character or the environment of the men which influences all the different manifestations of their activity. We must not expect the same degrees of solidarity in all the species of activity; there will be most of it in those species where each individual is in close dependence on the actions of the mass (economic, social, political life); there will be less of it in the intellectual activities (arts, sciences), where individual initiative has freer play.[215] Documents mention most habits (beliefs, customs, institutions) in the lump, without distinguishing individuals; and yet, in one and the same society, habits vary considerably from one man to another. It is necessary to take account of these differences, otherwise there is a danger of explaining the actions of artists and men of science by the beliefs and the habits of their prince or their tradesmen. (2) In order to ascertain the causes of an evolution, it is necessary to study the only beings which can evolve--men. Every evolution has for its cause a change in the material conditions or in the habits of certain men. Observation shows us two kinds of change. In the one case, the men remain the same, but change their manner of acting or thinking, either voluntarily through imitation, or by compulsion. In the other, the men who practised the old usage disappear and are replaced by others who do not practise it; these may be strangers, or they may be the descendants of the first set of men, but educated in a different manner. This renewing of the generations seems, in our day, to be the most active cause of evolution. It is natural to suppose that the same holds good of the past; evolution has been slower, the more exclusively each generation has been formed by the imitation of its forerunners.