But although the monasteries had so large a share in the work of the preservation of literature, the monks themselves wrought harm as well as good, for in their zeal to record sacred compositions they frequently destroyed older and often more valuable documents by scraping off the original writing and substituting other. This was done for economy's sake, when writing material was costly, and parchments thus treated are known as palimpsests. Owing to this reprehensible practice, many literary treasures have been irretrievably lost. Our Anglo-Saxon literature, for instance, is not represented by any contemporary copies. The Anglo-Norman writers had a contempt for the old English manuscripts, and turned them into palimpsests without the slightest idea that there could be any value in them, and attached far more importance to the writing they themselves were about to make. Thus it happens that we are in the same position with regard to Anglo-Saxon literature as with regard to classical authors. No original documents exist, and it is known to us solely through copies, single copies, in most cases. Beowulf, for instance, is represented only by a manuscript of the first half of the eleventh century, and Caedmon by a manuscript of the tenth century. With the invention and spread of the knowledge of printing, however, the risk of loss was greatly reduced. Such ancient writings as came into the printer's hands were given a fresh lease of life which in many cases was of indefinite length, or rather, of practically eternal duration. But the fact of being printed was not invariably a safeguard. Some of the works of the early printers have disappeared completely, and many are represented only by single copies. The strange history of the British Museum copy of the famous _Book of St Albans_, will serve to show the vicissitudes with which the relics of the past have to contend in their journey down the ages.