Sec. 17. These early churches have been treated at some length, because they contain certain essential elements of planning in a state of probation. The basilican plan was doubtless the ideal of English builders during the sixth and early seventh centuries, but an ideal which was hard to compass where good building material was not plentiful. Thus Augustine and his companions contented themselves in most instances with a plan which recalled the aisled basilica, without following out its more elaborate details. It is remarkable that they should have departed from the usual Roman custom, and made their chancels at the east end of their churches: it is also remarkable to find at St Pancras the western porch, the origin of which appears to be the non-Roman _narthex_. Models existed, no doubt in the ruins of the Romano-British churches, which they repaired; and we have seen that at Silchester there is a regular _narthex_, while, on the other hand, there is a western apse. These models, however, were probably all of one general type, in which the chancel end was formed by an apsidal projection. When Roman Christianity reached the north, it had to contend with the efforts of Celtic missionaries; and those efforts were not met by it effectively until, in 664, the energetic leadership of Wilfrid secured a triumph for his party at the council of Whitby. Of the Celtic churches of the north we know but little: it seems likely that they were for the most part plain oratories of stone or wood, with or without a separate chancel. The simplest form, obviously, which a church can assume is a plain rectangle with an altar at one end. As the desirability of a special enclosure for the altar is recognised, a smaller rectangle will be added at the altar end of the main building, and so the distinction between nave and chancel will be formed. There are indications of this natural growth of plan in some of the early religious buildings in Ireland. In remote districts, as in Wales, the simple nave and chancel plan is general all through the middle ages; and the smaller country churches often follow the common Celtic plan of a single rectangle with no structural division. The ruined chapel at Heysham in Lancashire, a work of early date, is an undivided rectangle in plan. This is the form which would suggest itself naturally to the unskilled builder: the division of nave and chancel into a larger and smaller rectangle is the next step which would occur to his intelligence in the ordinary course of things. It is possible that Wilfrid and Benedict Biscop found that their aims would be best served by adhering in certain instances to the familiar Celtic plan, and so, while they hired foreign masons and craftsmen to build and furnish their earlier churches, and to set the example of building stone churches after the manner of the Romans, they were careful to avoid the prejudice which insistence on a new plan would have excited. The simplicity, moreover,