Two manufactured articles are of especial interest to the archæologist: the stone axes and the pottery. They occur in every settlement. Stone is imperishable, and clay well fired lasts almost as well. They vary according to age, place, fashion, and conditions, and form the foundation for all comparative, "typological" study.[96] Their remains play the same part in archæology as the characteristic fossils, "_Leit-fossilien_," in paleontology, not only determining age but throwing light on the migrations, relations, life, and thought of their makers. The Neolithic period gained its name from the polished stone implements which then appeared. Paleolithic man had learned by long experience the value of flint as the best material for his tools. He had learned to chip and flake it; first by blows, then by pressure, until the Solutrean lance-heads or "points" showed a beauty of form and finish unsurpassed by the best craftsmen of any later date. He had learned to give it a fair cutting edge by small "retouches." It seems never to have occurred to him to grind or whet the edge of his tools. If the axe thickened rapidly from the edge and was somewhat like a wedge, it was a good remedy against the brittleness of the flint, its great defect; and he put the more strength into the blow. The extreme hardness of flint made polishing very difficult. Most utensils of daily use were not polished at all. Many of the beautiful daggers, genuine works of art, were finished by a uniform, fine flaking down to the close of the period. Flint implements were not polished in Italy, Greece, Spain, and large parts of eastern Europe;[97] they increase in abundance in Scandinavia and England. Other kinds of less brittle but somewhat softer rock were generally used for polished axes. During the upper Paleolithic period, especially in the Magdalenian Epoch, daggers, lance-heads, awls, and needles were made of bone. For pointed implements, flint, while sometimes used, was far less suitable, except when the point was very short, as in engraving and carving tools. These bone implements were scraped into shape and often well smoothed. It seems but a step from smoothing a bone to polishing the edge of an axe, if not of too hard rock. But the chipped flint axe was very good, and they were accustomed to it. Forrer thinks that the change must have been made where flint was scarce and pebbles abundant.[98] In Scandinavia the kitchen-midden period was followed by an "arctic" culture, so called because of its distribution in the far north. Here we find implements of slate or schist polished only along the edges. This seems like a very natural intermediate stage. We do not know just where those attempts were first made. They may have been made at different points in Asia and Europe and at different times, and thus there may have been several independent centres of discovery and of radiation.