As the number of travelling merchants from various cities increased on its shores, it was natural that they should hold together more and more in a tacit offensive and defensive alliance against the aliens, and that when they returned home from their voyages they should speak of the mutual benefits rendered and the help that lay in union. Some influential persons among them doubtless brought pressure to bear upon the rulers and magistracies of the various cities to give their informal union an official character. Thus much is certain, that after a time the merchants from various cities who traded with the Baltic had united into a federation having a common seal and conforming to a common law, so that by the middle of the thirteenth century the Hanseatic League was practically consolidated, although this name for the association only occurs later. So far, however, the Union only exercised rights abroad. It was from Wisby also that the reaction was to come for union at home; but this was a little later, when its strength was well matured and established. What really, in the first instance, led the Germans from their inland towns to the shores of the Baltic was the desire to benefit by the great wealth that lay hidden in its waters in the form of fish, which could be obtained in return for the mere labour of fishing. At a time when all Europe was Catholic, or of the Greek Church, and fasts as well as feasts strictly observed, the sale of fish was an important industry, and, above all, of salted fish, since our forefathers were ignorant of the art of preserving these creatures fresh by means of ice. Now, from the beginning of the twelfth century until the beginning of the fifteenth, when they once more altered their course, each spring and autumn the migratory fish, and especially that most prolific and valued of fish, the herring, came in great shoals to the shores of Scania,[4] the isle of Rügen, and the coasts of Pomerania, tempting the inhabitants of the strand and near inland hamlets out on to the waters to secure these treasures. Nor had nature herewith ceased her bounties. At certain points of the littoral there were salt springs, in which the precious draught could at once be pickled; and it is certain that the art of preserving the gifts of the ocean from decay was familiar to the Slav inhabitants of these districts long before it was known to those of the German Ocean. Already, in the eleventh century, "salt Kolberg" was famed as an emporium for salted herrings; and the words of a Polish poem of rejoicing at a victory won over its inhabitants in 1105 are extant to this day. It has more historical than literary value. "Formerly," so jubilantly sang the conquerors of the harbour, "they brought us salt and stinking fish, now our sons bring them to us fresh and quivering."