From the NYT:
Conventional histories cite the mid-1700s as the turning point in the development of the modern adversarial system of justice in England and Colonial America, with defense lawyers and prosecutors facing off in court, Mr. Hitchcock and Mr. Turkel said. Their analysis tells a different story, however.
“Mapping all trials suggests that the real moment of evolution was in the first half of the 19th century,” with the advent of plea bargains that resulted in many more convictions, Mr. Hitchcock said. “The defendant’s experience of the criminal justice system changed radically. You were much more likely to be found guilty.”
…
[In the late 1700s] prison began to be used as an alternative to exile or capital punishment…. As Mr. Hitchcock said, “It’s hard to have plea bargaining when all they are going to do is hang you.”