On the still more important matter of the ordinary pay of the auxiliary regiments an almost equal uncertainty prevails. Our only two pieces of evidence on the subject, a passage in Tacitus and a phrase in Hadrian’s address to the garrison of Africa,[73] tell us nothing more than that the _equites cohortales_ were paid on a higher scale than the infantry, but received in their turn less than the _equites alares_, a preference in favour of the mounted men, which is not so great as appears at first sight, since it is clear that they were responsible for the upkeep of their own horses. The chief defect of these passages is that they do not mention the amount of the pay in any of the three cases. Our only basis for calculation is the fact that a legionary considered it promotion to be made _duplicarius alae_; hence the pay of an ordinary cavalryman must have been more than half that of a legionary. On _a priori_ considerations it can hardly have been less, if, as Hadrian’s speech suggests, he paid for his own arms and mount, and if he also, like the legionaries, had the cost of his rations deducted from his pay. On the whole, however, it seems best to defer speculation until further evidence is forthcoming.[74] _Internal organization._ As is only natural in the case of a professional army with so long a term of service, the internal organization of the auxiliary regiments reveals a far more complicated system of grades and promotions than anything which the ancient world had yet known. The epigraphical evidence is abundant, and the efforts of modern scholars, particularly von Domaszewski in his monumental treatise, _Die Rangordnung des römischen Heeres_, have done much to make the main lines of the system clear. Difficulties in detail still remain, but we may hope for their ultimate solution. The commanding officer of an _ala quingenaria_ or _miliaria_, or of a _cohors quingenaria_ bore the title of _praefectus_. _Cohortes miliariae_ and the _cohortes civium Romanorum_, which occupied an exceptional position,[75] were commanded by _tribuni_. Early inscriptions also mention a _subpraefectus alae_ and a _subpraefectus cohortis_, but these posts seem later to have been abandoned.[76] In later times in case of the absence of the _praefectus_, his place seems to have been filled by an officer placed temporarily in charge with the title of _praepositus_ or _curator_. Questions concerning the order of precedence among the _praefecti_ and _tribuni_, and their place in the military hierarchy generally, are so closely connected with the method of selection and appointment of these officers at different periods, that they are best left for future discussion.[77] It is only important here to note that they usually entered the service with this rank, and that it is very rare to find the regular commander either of an ala or cohort drawn from among the lower officers.