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But to start at the beginning... According to The Divine Plan of Ages (Studies in The Scriptures, part I (1886), pages 253-4):
[...] Rome, fourth, the strong kingdom, the iron legs and clay-mixed feet... The iron empire, Rome, was by far the strongest, and endured longer than its predecessors. In fact, the Roman Empire still continues, as represented in the nations of Europe. This division is represented in the ten toes of the image. The clay element blended with the iron in the feet represents the mixture of church and state. This mixture is in the Scriptures termed “Babylon”—confusion. As we shall presently see, stone is the symbol of the true Kingdom of God, and Babylon substituted an imitation of stone -clay— which it has united with the fragmentary remains of the [iron] Roman Empire.