Modern inquiries into the history of the Roman army have elucidated the fact that there were two periods in the evolution of the Roman recruiting-system, whether of the Roman legions or of the auxiliary troops. The first period embraced the first two centuries of the Roman Empire, while the second began with Diocletian. The most characteristic features of the first period were as follows: recruiting of legions among the town population or territories attributed to a town; recruiting of auxiliary troops chiefly among the population of non-municipal territories; prevalence of the idea of conscription, which remained as a matter of fact chiefly theoretical as far as the legions were concerned, the latter consisting chiefly of volunteers; granting of Roman citizenship to soldiers who were serving in the legions at the moment of their being enrolled, and the receiving of Roman citizenship by the soldiers of auxiliary troops after the completion of their period of service; absence of any form of compulsion in the recruiting system. The second period presents entirely different features. The distinction between legions and auxiliary troops vanished almost completely, as both were recruited chiefly among the rural population; for the idea of conscription as the fulfilment of the duty of citizenship was substituted either the idea of military service for money, the idea of mercenary troops, or the idea of compulsory military service, this service being treated in the same way as the compulsory levying of taxes.