When we consider the enormous number of manuscript books that must have existed in Europe in the middle ages, we may well wonder why they have become relatively rare in modern times. Several explanations account for this. In the first place, the practice of erasing old manuscripts and using the same vellum again for other works was extremely common. Secondly, vast numbers of manuscripts in the monasteries and other libraries of Europe were wantonly or accidentally destroyed by fire, especially in times of war and religious fanaticism. In the third place, the early binders, down through the sixteenth century and even later, used sheets of vellum from old manuscripts for the linings and the covers of printed books. Finally, after the invention of printing, as soon as a given work had been adequately and handsomely printed in a standard edition, all but the finest manuscripts of that book would naturally be looked upon as of little value, and would be subject to loss and decay if not to deliberate destruction. Owing to these and perhaps other causes it is almost entirely the religious manuscripts that have survived, except those preserved in royal libraries and museums from the finer collections of the middle ages. The invention of printing was not the work of any one man. Not only were printed pages of text with accompanying pictures produced from woodcut blocks in Holland a quarter of a century before Gutenberg began his work at Mainz, but it is pretty well established that movable types were employed by Laurence Koster, of Haarlem, as early as 1430. But Koster, who died about 1440, did not carry his invention beyond the experimental stages, and produced no really fine printing. Moreover, his work had no immediate successor in Holland. Whether it be true, as sometimes alleged, that Gutenberg first learned of the new art from one of Koster's workmen, we have no means of knowing. At any rate, Gutenberg's contemporaries as well as his successors gave to him the credit of the invention. That he was not the first to conceive the idea of multiplying impressions of type-forms by the use of a screw press is evident; but he was the first to develop the invention to a point where it became capable of indefinite extension. He seems to have worked in secret for some years on the problems involved in type-founding and printing before the year 1450, when he set up his shop in Mainz.