The last of the three stages, stone, bronze, iron, which mark the prehistoric development of most now civilised peoples; these, of course, occurred at different periods, and were of different duration in different cases; they are named from the material employed in making cutting instruments and weapons; the forms of instruments are freer than in the bronze period, and rectilineal gives places to free curvilineal decoration; this age is marked, too, by the introduction of writing and the beginning of literary and historic records. See Ages .