In United States v. Gooding (1827) 25 U.S. 460, the admissibility of co-conspirators' statements was first established, interpreting statements of co-conspirator as res gestae and thus admissible against defendant, and the Court has repeatedly reaffirmed the exception as accepted practice.
Justice Story said:
"In general, the rules of evidence in criminal and civil cases are the same. Whatever the agent does, within the scope of his authority, binds his principal, and is deemed his act. It must, indeed, be shown that the agent has the authority, and that the act is within its scope; but these being conceded or proved, either by the course of business or by express authorization, the same conclusion arises, in point of law, in both cases. Nor is there any authority for confining the rule to civil cases. On the contrary, it is the known and familiar principle of criminal jurisprudence, that he who commands or procures a crime to be done, if it is done, is guilty of the crime, and the act is his act."
"In general, it may be said, that it is sufficient certainty in an indictment, to allege the offence in the very terms of the statute. And in endeavors to commit a revolt, which is by statute in England made a capital offence, it has always been deemed sufficient, to allege the offence in the words of the statute, without setting forth any particulars of the manner or the means."
"The case of treason stands upon a peculiar ground; there, the overt acts must, by statute, be specially laid in the indictment, and must be proved as laid. The very act, and mode of the act, must, therefore, be laid as it is intended to be proved."