Adoption in Rome was in the form of patronage from an old, wealthy man to a young upcoming youth. When that youth advanced in office and power, he would in turn repay his benefactor.

The late Republic and the Empire deviated heavily from the old Roman ways of honour, earning office through valour and military commmand and so on. The fall of Rome and all the violent rebelions can be partially attributed to the corrupt and inept power hierarchy in the late empire.

Obviously in feudal Europe this was not the way to do it. Bloodlines were important, as the nobility set themselves apart from the yeomanry and common folk not just by material wealth but also by blood.