The Republic by Plato Part 8 Page 14

different ways: the iron and brass fell to acquiring money and land and houses and gold and silver; but the gold and silver races, not wanting money but having the true riches in their own nature, inclined towards virtue and the ancient order of things.

There was a battle between them, and at last they agreed to distribute their land and houses among individual owners; and they enslaved their friends and maintainers, whom they had formerly protected in the condition of freemen, and made of them subjects and servants; and they themselves were engaged in war and in keeping a watch against them.

I believe that you have rightly conceived the origin of the change.

And the new government which thus arises will be of a form intermediate between oligarchy and aristocracy?