71.—In any proceedings for an offence under this Act, it shall not be necessary to—

(a) negative by evidence the existence of any order under section 24 or 25 of this Act or any licence or permission under this or any other Act,

(b) prove that any act which is the subject of the proceedings was not caused by or the result or effect of, or was not consequent upon any other thing lawfully done,

and the onus of proving such licence or permission or that such act was so caused or was such a result or effect or was so consequent upon shall be on, in the case of such a licence or permission, the person seeking to avail himself thereof, or in any other case, the person against whom the proceedings are instituted.