When the theory that block-books were "Steps towards the Invention of Printing" is thus opposed by the rival theory that they were forerunners of stereotyped plates, we are left free to consider, uncoerced by supposed necessities, such evidence as exists as to the dates of the specimens of block-printing still extant. Putting aside the late Italian block-book as a mere survival, we find two[7] broadly distinguished groups, one earlier, the dates of members of which can only be conjectured, the other later, several of which can only be definitely connected with the years 1470 to 1473. The characteristics of the earlier group are that they are printed (1) with a watery brown ink; (2) always on one side of the paper only; (3) without mechanical pressure;[8] (4) two consecutive pages at a time, so that they cannot be arranged in quires, but must be folded and stitched separately, and the book thus formed[9] begins and ends with a blank page and has a pair of blank pages between each pair of printed ones. This arrangement in some extant copies has been altered by modern binders, who have divided the sheets, mounted each leaf on a guard, and then gathered them, at their own will, into quires. The inconvenient intervention of the blank pages has also sometimes been wrestled with (at an early date) by gluing the leaves together, so that all the leaves, except the first and last, are double, and the printed pages follow each other without interruption. These expedients, however, are easily detected, and the original principle of arrangement is free from doubt. In the later block-books, on the other hand, we note one or more of the following characteristics: (1) the use of the thick black ink (really a kind of paint) employed in ordinary printing; (2) printing on both sides of the paper; (3) marks of pressure, showing that the paper has been passed through a printing-press; (4) the arrangement of the blocks in such a way as to permit the sheets to be gathered into quires.