In an action on an express contract for commissions for procuring purchasers of land, the defendant was entitled under a general denial to offer testimony, not only denying the contract as claimed, but also proving the contract which they in fact made, and non-performance thereof by the plaintiff. Tracey Land Co. v. Polk, etc. Co., 131 Iowa, 40, 107 N. W. 1029; Harris v. Moore, 134 Iowa, 704, 112 N. W. 163; Weaver v. Richards, 156 Mich. 320. Compare Sec. 682.