31. The world power in its totality appears as a colossal human form: Babylon the head of gold, Medo-Persia the breast and two arms of silver, Græco-Macedonia the belly and two thighs of brass, and Rome, with its Germano-Slavonic offshoots, the legs of iron and feet of iron and clay, the fourth still existing. Those kingdoms only are mentioned which stand in some relation to the kingdom of God; of these none is left out; the final establishment of that kingdom is the aim of His moral government of the world. The colossus of metal stands on weak feet, of clay. All man's glory is as ephemeral and worthless as chaff (compare 1Pe 1:24). But the kingdom of God, small and unheeded as a "stone" on the ground is compact in its homogeneous unity; whereas the world power, in its heterogeneous constituents successively supplanting one another, contains the elements of decay. The relation of the stone to the mountain is that of the kingdom of the cross (Mt 16:23; Lu 24:26) to the kingdom of glory, the latter beginning, and the former ending when the kingdom of God breaks in pieces the kingdoms of the world (Re 11:15). Christ's contrast between the two kingdoms refers to this passage.
      a great image--literally, "one image that was great." Though the kingdoms were different, it was essentially one and the same world power under different phases, just as the image was one, though the parts were of different metals.

JFB.