An offer which is to be accepted by doing a certain act or series of acts may be revoked before any of such acts are done.1 After all the acts which are prescribed by the offer have been performed, the contract is completed by the acceptance of the offer, and revocation is thenceforth impossible.2 The difficulty in cases of this sort is found where the offeree has performed a part, but not all, of the acts prescribed by the offer; and the offeror thereupon attempts to revoke. At least three theories as to the right of the offeror to revoke in cases of this sort have been advanced. It has been urged that as the offer has not been accepted the offeror can revoke, subject to liability in quasi-contract for benefits received by him; but not subject to any liability for expenses incurred by the offeree, which have not conferred a benefit upon the offeror.3 This theory always surprises those who have never considered the question; and the result seems to most persons immoral and unethical. It has been suggested that the offeror should be estopped from revoking the offer.4 The difficulty in adopting this theory is that if it is conceded that apart from estoppel the offer is revocable, the only fact which calls for the application of estoppel is the breach of a gratuitous promise. It would simplify our law to make gratuitous promises enforceable in this way; and it might possibly improve it; but no reason appears for applying estoppel to this type of gratuitous promise, but not to other types, or indeed, for using estoppel at all as an excuse for reaching the desired result. The third theory that has been advanced is that such offer contains an implied promise not to revoke for a reasonable time or for the time fixed by the offer, in consideration of the offeree's beginning performance in a reasonable time or in the time fixed by the offer, as the case may be.5 The third theory seems to represent the exact understanding of the parties. If this term were inserted in the offer by express words, there can be no doubt that on commencement of performance of the prescribed acts within the prescribed period, the offer would be irrevocable. On the other hand, if the offeror had reserved to himself the right to revoke the offer at any stage of performance prior to complete performance, no one could complain if he exercised such right, although it seems likely that few offerees would accept on such terms. There is some authority for holding that such offer implies a promise not to revoke, the consideration for which is beginning performance.6 A reward offered by a county under statutory authority for the arrest of a criminal within a certain limited time, is said to imply a contract with anyone who begins performance, that if he succeeds ultimately in performing the acts called for in such offer, he shall receive such reward.7

3 Moffett, Hogkins and Clarke Co. v. Rochester, 178 U. S. 373, 44 L. ed. 1108; St. Nicholas Church v. Kropp, 135 Minn. 115, L. R. A. 1917D, 741, 160 N. W. 500.

1 Shuey v. United States, 92 U. S. 73, 73 L. ed. 23.

2 Bishop v. Eaton, 161 Mass. 406, 42 Am. St. Rep. 437, 37 N. E. 665.

3 Wald's Pollock on Contracts (Williston's Notes), p. 34, note 39. See also, True Conception of Unilateral Contracts, by I. Maurice Wormser, 26 Yale Law Journal, 136.

4 Offers Calling for a Consideration other than a Counter-promise, by Clarence D. Ashley, 23 Harvard Law Review, 159.

The effect of an attempted revocation on the part of the offeror is a question which presents no difficulty under the first theory, since it is assumed that the offeror may revoke such offer without any liability other than that in quasi-contract. If the doctrine of estoppel is to be applied under the second theory, it would seem as though such revocation would be inoperative by reason of such estoppel. Under the third theory the question of the effect of such attempted revocation presents greater difficulties. By the terms of the offer, acceptance is not complete until full performance of all the acts stipulated for in the offer; and the offeree may stop performance without any liability on his part. Under the third theory must the offeree continue to perform in spite of the offerors notice of revocation; or has he the choice to continue such performance if he chooses, or at his option to sue at once for damages; or is he obliged to desist from further performance and sue for damages In an offer of a promise for a promise the making of the promise of acceptance is an instantaneous transaction; and the contract is not complete until such acceptance.8 If the contract is made, and it is then broken by one of the parties, it is the duty of the party not in default to do everything that a reasonable and prudent man would do to mitigate damages; and he can not continue performance in order to increase the amount of damages.9 The offer may by its terms be one which is to be accepted by doing an act which is instantaneous and the performance of which does not involve any appreciable interval of time. In cases of this sort there can be no revocation during the performance of the act. The offer may be revoked before performance;10 and after performance the contract is complete.11 An illustration of this sort is a reward for disclosing information which the offeree already has.12 The practical difficulty is found in cases of an offer which can be accepted only by the performance of a series of acts, extending over an appreciable interval of time. This difficulty with the contract in which a promise is offered for an act arises out of the fact that the performance of the act is acceptance, consideration, and performance at one and the same time; and that it is therefore impossible in case of an attempted revocation by the offeror to apply at the same time the principles which govern acceptance and the principles which govern performance.

5 Irrevocable Offers, by D. O. Mc-Govney, 27 Harvard Law Review, 644.