Another class of churches forms a distinctly later type than the above simple quadrilateral structures. These are the churches consisting of a nave and chancel. Not that the method of construction or the details of these churches show any advance on the previous class. On the contrary, the details are in many cases as simple and rude as those of the one-chambered churches; but the alteration of the ground plan, by the addition of a separate chancel, shows a development of the religious service, leading to the inference that the type of churches with chancels is later than the single-chambered ones. This, however, only shows that the idea is later, not that single-chambered churches did not continue to be erected after the chancel had, in some instances, been introduced.[44] The persistence of an original form of plan is remarkable and is well exemplified in the history of the castles of Scotland, which shows how the primitive keep-plan of the thirteenth century continued to be adopted up to the seventeenth century, long after other and more developed forms of castles had been introduced. The tendency in churches, however, seems to have been to adhere to the chancel plan after its introduction, and even to alter older simple churches by the addition of a chancel to one-chambered structures. Of this we have mentioned an instance in Ireland at St. Kevin’s oratory at Glendalough, and we shall meet with examples in Scotland as we proceed. In other instances, primitive oratories have been converted into churches with chancels by the addition of a nave, the original oratory being retained as the chancel. We have thus a transitional plan forming the link which connects these primitive single-chambered churches with the more advanced type of church with nave and chancel. In most of these early churches the chancel forms a separate apartment from the nave, the entrance to the chancel being by a doorway only, generally similar in size and form to that of the western entrance to the nave. The chancel arch occurs, in some instances, as a later development. This, together with a few other details, seems to point to the influence of the Continental or Romanesque style which was slowly beginning to make itself felt in some parts of the country. All the above types of structures have been thoroughly examined and described by Mr. Muir, and will be more fully dealt with in the detailed descriptions of the churches derived from Mr. Muir’s works.