Occupied field doctrine requires state laws to yield to central law where irreconcilable conflict exists, subject to presidential assent. The doctrine of occupied field requires State legislation to yield where it irreconcilably conflicts with Central law within a common subject-matter; coexistence is permitted where both can operate without clash. State laws incidentally touching a central topic are not wholly prohibited, but a plain conflict will render the State statute subordinate unless presidential assent has been obtained to validate the repugnant enactment.