Administrative agencies like the Federal Trade Commission have never been restricted by the rigid rules of evidence. Interstate Commerce Commission v. Baird, 194 U. S. 25, 194 U. S. 44. And, of course, rules which bar certain types of evidence in criminal or quasi-criminal cases are not controlling in proceedings where the effect of the Commission's order is not to punish or to fasten liability on respondents for past conduct, but to ban specific practices for the future in accordance with the general mandate of Congress.