By a comparison of the evolutions of the different species of facts which coexist in one and the same society, the "historical" school was led to the discovery of solidarity (_Zusammenhang_).[213] But, before attempting to discover its causes by analysis, the adherents of this school assumed the existence of a permanent general cause residing in the society itself. And, as it was customary to personify society, a special temperament was attributed to it, the peculiar genius of the nation or the race, manifesting itself in the different social activities and explaining their solidarity.[214] This was simply an hypothesis suggested by the animal world, in which each species has permanent characteristics. It would have been inadequate, for in order to explain how a given society comes to change its character from one epoch to another (the Greeks between the seventh and the fourth centuries, the English between the fifteenth and the nineteenth), it would have been necessary to invoke the aid of external causes. And the theory is untenable, for all the societies known to history are groups of men without anthropological unity and without common hereditary characteristics. In addition to these metaphysical or metaphorical explanations, attempts have been made to apply to the investigation of causes in history the classical procedure of the natural sciences: the comparison of parallel series of successive phenomena in order to discover those which always appear together. The "comparative method" has assumed several different forms. Sometimes the subject of study has been a detail of social life (a usage, an institution, a belief, a rule), defined in abstract terms; its evolutions in different societies have been compared with a view to determine the common evolution which is to be attributed to one and the same general cause. Thus have arisen comparative philology, mythology, and law. It has been proposed (in England) to give precision to the comparative method by applying "statistics"; this would mean the systematic comparison of all known societies and the enumeration of all the cases where two usages are found together. This is the principle of Bacon's tables of agreement; it is to be feared that it will be no more fertile in results. The defect of all such methods is that they apply to abstract and partly arbitrary notions, sometimes merely to verbal resemblances, and do not rest on a knowledge of the whole of the conditions under which the facts occur.