In a civil action, a presumption imposes on the party against whom it is directed the burden of proving the nonexistence of the presumed fact. If the party against whom the presumption is established fails to offer evidence showing the nonexistence of the presumed fact, the trier shall accept the existence of said fact. If evidence is introduced in support of a determination as to the nonexistence of said fact, the party wishing to rebut the presumption shall persuade the trier that nonexistence of the presumed fact is more likely than its existence.