2. It is a general rule, that on any issue it is sufficient to prove the substance of the issue. For example, in a case where the defendant pleaded payment of the principal sum and all interest due, and it appeared in evidence that a gross sum was paid, not amounting to the full interest, but accepted by the plaintiff as full payment, the proof was held to be sufficient. 2 Str. 690; 1 Phil. Ev. 161.