To make even a rough estimate of their relative importance by any method more or less statistical, is evidently impossible. One single war--we all know--may be productive of more evil, immediate and subsequent, than hundreds of years of the unchecked action of the mutual-aid principle may be productive of good. But when we see that in the animal world, progressive development and mutual aid go hand in hand, while the inner struggle within the species is concomitant with retrogressive development; when we notice that with man, even success in struggle and war is proportionate to the development of mutual aid in each of the two conflicting nations, cities, parties, or tribes, and that in the process of evolution war itself (so far as it can go this way) has been made subservient to the ends of progress in mutual aid within the nation, the city or the clan--we already obtain a perception of the dominating influence of the mutual-aid factor as an element of progress. But we see also that the practice of mutual aid and its successive developments have created the very conditions of society life in which man was enabled to develop his arts, knowledge, and intelligence; and that the periods when institutions based on the mutual-aid tendency took their greatest development were also the periods of the greatest progress in arts, industry, and science. In fact, the study of the inner life of the medieval city and of the ancient Greek cities reveals the fact that the combination of mutual aid, as it was practised within the guild and the Greek clan, with a large initiative which was left to the individual and the group by means of the federative principle, gave to mankind the two greatest periods of its history--the ancient Greek city and the medieval city periods; while the ruin of the above institutions during the State periods of history, which followed, corresponded in both cases to a rapid decay.