(5) In various other ways, apart from the mutual influence of divergent group-customs, the progress of civilization tends to produce variations in ideals. The increase of knowledge, the development of science and philosophy, bring floods of new ideas to burst the old dams; deepening insight reveals the irrationality of old ideas to the leaders of thought. The progress of the arts gives new interests and valuations. The spiritual seers and prophets see visions of a better order and proclaim new gospels. The development of classes and castes allows to the aristocracy more leisure to think and criticize; the institution of slavery, in particular, produced a class of slave-owners with ample time to dissect their inherited conceptions. (6) Finally, where, under favoring conditions, the danger of war in which man has for the most part lived became less acute, custom generally grew laxer. It is the imperious necessity of selfpreservation that has been the greatest conservative force; warlike states have demanded strict allegiance and looked with suspicion upon deviations from the group ideals. But peoples that, whether from a fortunate geographical situation or because of their marked superiority in numbers and power over their neighbors have escaped this need of perpetual self-defense could afford to relax their vigilance for conformity. And the very notable increase in individual variations in conduct and ideal during the past century has been largely owing to the era of comparative peace. We seem to be reaching the age when the advantage is to lie not with the nation that has the most rigid customs, but with the nation that shows the most individual initiative and progress. Conservatism vs. radicalism