Matter in mitigation of damages is a defense, R.C.M. 1947, [8] sec. 93-3411, and the burden of pleading and proving it rests with the defendant. Compare Miller v. Yellowstone Irr. Dist., 91 Mont. 538, 9 P.2d 795; McKim v. Beiseker, 56 Mont. 330, 185 P. 153. There was nothing in the record from which the jury could have [9] fixed a value on the salvage and in consequence the court did not err in giving the instruction on damages without any reference to the question of the salvage.