Jamieson, Fausset and Brown on Daniel 2:41-43

41-43. feet ... toes ... part ... clay ... iron--explained presently, "the kingdom shall be partly strong, partly broken" (rather, "brittle," as earthenware); and Da 2:43, "they shall mingle ... with the seed of men," that is, there will be power (in its deteriorated form, iron) mixed up with that which is wholly of man, and therefore brittle; power in the hands of the people having no internal stability, though something is left of the strength of the iron [Tregelles]. Newton, who understands the Roman empire to be parted into the ten kingdoms already (whereas Tregelles makes them future), explains the "clay" mixture as the blending of barbarous nations with Rome by intermarriages and alliances, in which there was no stable amalgamation, though the ten kingdoms retained much of Rome's strength. The "mingling with the seed of men" (Da 2:44) seems to refer to Ge 6:2, where the marriages of the seed of godly Seth with the daughters of ungodly Cain are described in similar words. The reference, therefore, seems to be to the blending of the Christianized Roman empire with the pagan nations, a deterioration being the result. Efforts have been often made to reunite the parts into one great empire, as by Charlemagne and Napoleon, but in vain. Christ alone shall effect that.