In an action by a real estate broker for commissions, where the answer is a general denial, the proof is restricted to sustaining or controverting the facts stated in the petition, and defendant cannot show that a third party and not the plaintiff introduced the purchaser to the defendant. St. Felix v. Green, 34 Neb. 800, 52 N. W. 821; Reese v. Garth, 36 Mo. App. 641.