In the mid-nineteenth century, the standard for conviction in murder cases was proof "to an absolute certainty" that the dead body was that of the victim, or absolute proof of corpus delicti.
John Webster, accused of killing George Parkman, was one of the first defendants to be convicted without absolute evidence that the victim had been murdered. It could not be established that the bones discovered were Parkman's. Judge Lemuel Shaw opened the door for the jury to convict anyway by changing the standard. He instructed them that they need only prove corpus delicti "beyond a reasonable doubt."
Present-day prosecutors have many more tools for proving guilt -- including DNA analysis, lie detectors, ballistics tests, and fingerprinting.
Which legal standard of guilt do you think is more appropriate in capital murder cases today?