The same system of blear-eyed officialism had found a still more ingenious method of throttling the society which it was endeavouring to nurse back into infancy. It was under Severus Alexander (about A.D. 230) that the various _collegia_ or guilds were incorporated by charter, so that every industry whatever became a close corporation. This rendered the task of administration much simpler. It meant that every human occupation became hereditary. There was, for example, a guild of the _coloni_ or tillers of the soil. The most benevolent of the emperors, Marcus Aurelius and the two Severi, had planted barbarians on Roman soil under condition of military service in lieu of rent. This service became hereditary also. Before long each piece of ground had to supply a recruit. The _decuriones_, moreover, or municipal senators, who had once been the honoured magistrates of their townships, also became a caste. As they were made responsible for the collection of property tax in their boroughs, and as wealth began to decline and taxation to increase, they were reduced to a condition of penury and misery. The exemption from taxation of whole classes of society, such as the soldiers and eventually the Christian clergy, added to their burdens. Then, since many of them attempted to evade the distresses entailed upon their rank by joining the army or even selling themselves into slavery, a decree was issued which made their office hereditary. It became a form of punishment to enrol an offender among these _curiales_. A decree of Constantine bound all the tillers of the soil in hereditary bondage for ever. In these ways Roman society fell into stagnation. Since the progress of the Manchurian Empire in China proceeded on very similar lines, it looks as if the benevolent despotism engendered by highly centralised government of very large areas was one of the methods by which Providence is accustomed to bring great empires low.