There can be no question of experimenting in sociology.[242] Not that we cannot act upon the social phenomena: they are, on the contrary, the most modifiable of all. But an experiment properly so-called consists in comparing two cases which differ from each other by a certain definite circumstance, and by that one alone. We have no means of determining two cases of this kind in sociology. It is true that in the absence of direct experiments nature presents indirect ones. They are the pathological cases, unfortunately too frequent in the life of societies, the more or less serious perturbations which they undergo through accidental or passing causes. Such are the revolutionary periods which correspond to diseases in living bodies. If we properly extend Broussais’ principle to sociology, that is to say, if we admit that morbid phenomena are produced by the effect of the same laws as normal phenomena, then social pathology will in some measure replace experiments. It will be said that this study has been fruitless up to the present time. But the reason of this is, according to Comte, that direct or indirect experimenting ought, like simple observation, to be subject to rational conceptions. Both are only productive in a sociology already possessed of its essential laws. The comparative method, so useful to the biologist, is also precious for the sociologist. It draws together the various states of human society which coexist on the different parts of the earth’s surface, and among peoples independent of one another. Undoubtedly, if the total development only is considered, the evolution of Humanity is one. It nevertheless remains true that very considerable and very varied populations have as yet only reached the more or less inferior degrees of this evolution. We can thus observe them simultaneously and compare their successive phases. From the Fuegians to the most civilised nations in Europe, we can imagine no “social shade” which is not at present realised on some portion of the globe. Frequently, within the same nation, the social condition of the various classes represents states of civilisation which are very far removed from one another. Paris to-day contains more or less faithful “survivors” of nearly all the anterior degrees of social evolution, especially from the intellectual point of view.[243] This comparative process holds good for social statics as for social dynamics. Even in statics a comparison can be established between animal societies and human society.