The directors of a domestic mutual insurer may apportion and pay dividends to its members as entitled thereto, but only out of that part of its surplus funds which is in excess of its required minimum surplus and which represents net realized savings and net realized earnings from its business. Any classification of its participating policies and of risks assumed thereunder which the insurer may make must be reasonable. No dividend may be paid which is inequitable or which unfairly discriminates as between the classifications or as between policies within the same classification. No dividend, otherwise earned, may be made contingent upon the payment of a renewal premium on any policy.