So far, therefore, as negative evidence goes, it is complete in showing that our megalithic circles did not exist in the time of the Romans, and that they were not temples. Unfortunately, however, no amount of negative evidence is sufficient to prove an affirmative, though it may suffice to establish a strong presumption in favour of a particular view, and, at all events, clears the way for the production of any direct evidence which we may have. The direct written evidence that has been adduced is, however, of the most shadowy character. It amounts to little more than this:--that every allusion to these monuments in mediæval authors, every local tradition, every scrap of intelligence we have regarding them, points to a post-Roman origin. No writer, of any age or country, suggested their being prehistoric or even pre-Roman before the age of Stukeley,--say 1700. There is, so far as I know, only one paragraph in any classical author which mentions a French or British temple; but it belonged to so exceptional a community that it would hardly be safe to base an argument upon it. A "hieron," Strabo tells us, existed at the mouth of the Loire, inhabited by a colony of women who lived apart from their husbands, but the roof or thatch of the roof of whose temple was renewed annually:[24] a fact that shows, in the first place, that it had a roof, and in the second, that it was not a very dignified or permanent structure. It would add very much to the clearness of our conception on this subject if the early Christian writers had left us some descriptions of the temples of the Britons when the missionaries first came among them. Though not quite so silent on the subject as the classical authors, their direct evidence is far from being so complete as might be wished. One of the passages most distinctly bearing on this question is found in a letter which Pope Gregory the Great addressed to the Abbot Millitus, then on a mission to England. In this letter he instructs him by no means to destroy the temples of the idols belonging to the English, but only the idols which are found in them; and adds, "Let holy water be made, and sprinkled over them. Let altars be constructed, and relics placed on them; insomuch as if these temples are well constructed, it is necessary that they should be converted from the worship of dæmons to the service of the true God. So that the people, seeing their temples are not destroyed, may put away errors from their hearts, and, acknowledging the true God and adoring Him, may the more willingly assemble in the places where they were accustomed to meet."[25] A little further on he adds, in order that no apparent change may be made, "that on great festivals the people may erect huts of boughs around those churches which have been converted (commutatæ) from temples."