The second half of the nineteenth century constitutes a fourth period. Those who dissented from the Liberalism of the previous epoch are responsible for the schisms that began to manifest themselves in four different directions at this time. The Historical school advocates the employment of the inductive method, and the State Socialists press the claims of a new social policy. Marxism is an attack upon the scientific basis of the science, and Christian Socialism a challenge to its ethical implications. A fifth epoch comprises the end of the nineteenth century and the beginning of the twentieth. The heading “Recent Doctrines” includes several theories that are already well known to us, but which seem transfigured—or disfigured, as some would prefer to put it—in their new surroundings. The Hedonistic doctrine and the theory of rent represent a kind of revision of the Classical theories. Solidarism is an attempt to bridge the gap that exists between individualism and socialism, whilst Anarchism can only be described as a kind of impassioned Liberalism. This order of succession must not be taken to imply that each antecedent doctrine has either been eliminated by some subsequent doctrine or else incorporated in it. The rise of the Historical school in the middle of the nineteenth century, for example, happened to be contemporaneous with the triumph of the Liberal school and the revival of Optimism. In a similar fashion the new Liberalism of the Austrian school was coincident with the advent of State intervention and the rise of Collectivism. We cannot, however, help noticing a certain rhythmical sequence in this evolutionary process. Thus we find the Classical doctrine, as it is called, outlined in the earliest draft of the science, but disappearing under the stress of more or less socialistic doctrines, to reappear in a new guise later on. There is no necessity for regarding this as a mere ebb and flow such as distinguishes the fortunes of political parties under a parliamentary _régime_. Such alternation in the history of a doctrine has its explanation not so much in the character of the doctrine itself as in the favour of public opinion, which varies with the fickleness of the winds of heaven.