How Did the Romans Develop Into Their Own Equestrian Order?

In 129 b c, a law was passed to say that you could not be an qestrian and a senator at the same time. So this separated senators from the equites completely. If you were going to enter the senate, you had to surrender your public horse before taking up membership of that body. We have a nice account from plutarch in his biography of pompey, which talks about when he had to surrender his horse to enter the Senate.