It seems impossible on theory sueessfully to question the power of one who offers to enter into a unilateral contract to withdraw his offer at any time until performance has been completed by the offeree, though obvious injustice may arise in such a case. For instance, if A offers one hundred dollars if B will complete a piece of work, and B sets about the work and nearly finishes it, it is a hardship upon B if while the work is still incomplete, A may revoke his offer. Yet any other result involves either a violation of recognized principles of contract, or the invention of new ones. To say that the beginning of work by B amounts to an assent binding both A and B to the performance and payment is to change the hypothesis that A offered, not to make a bilateral contract, but a unilateral one, and in effect to deny the right of an offeror to dictate the terms of his offer. Doubtless wherever possible, as matter of construction, a court would and should construe an offer as contemplating a bilateral rather than a unilatera contract; since in a bilateral contract both parties are pro tected from a period prior to the beginning of performancon either side.67 But the case supposed is one where the offer is so clearly for the formation of a unilateral contract, that no other reasonable construction is possible than that the offerer demands as an exchange for his promise, not a promise but a completed act. After the offeree has begun to perform under such an offer he may unquestionably stop performance halfway if he concludes that after all he does not care to enter into the contract, and if the offeror also may not revoke at that time he is bound by a promise for which he has not received, and may never receive, the consideration requested, since the whole transaction is still optional with the offeree. The suggestion has been made to avoid the hardship of denying relief to the offeree that if the consideration requested in an offer of a unilateral contract, will necessarily take time for its performance, the offer should be regarded as containing by implication a subordinate offer to hold the main offer open for a reasonable time in consideration of the beginning of performance of the offeree.68 This analysis finds some support in the English cases which hold that a collateral contract is formed by attending an auction sale,69 but is open to the criticism made of those cases; namely, that the necessary assumptions of fact are artificial. Moreover, if the doctrine is adopted, there seems no reason why the principle should not cover other cases. If beginning performance of an act requested, indicates assent to and constitutes the consideration for a contract to hold the offer open, it would seem that making preparation and taking trouble and expense preliminary to any requested performance would likewise create a similar contract, yet the contention could certainly not be maintained as a general principle.70 Indeed, if a col-

65 Shuey v. United States, 92 U. 8. 73, 23 L. Ed. 697. See also Sullivan v. Phillipe, 178 Ind. 164, 98 N. E. 868.

66 Hoggard v. Dickpraon, 180 Mo. App. 70, 165 S. W. 1136.

67 See Lennox v. Murphy, 171 Mass. 370, 50 N. E. 644; Laacellee v. Clark, 204 Mass. 362, 90 N. E. 875; Poet v. Frank, 132 N. Y. Supp. 807, 75 N. Y. Misc. 130; Senter v. Senter, 87 Ohio, 377, 101 N. E. 272. So a promise to convey the plaintiff a piece of land if he married the promisor's daughter, and was "good and kind" to her was interpreted as entitling the promisee on his marriage to a deed, which should contain & condition subsequent of goodnera and kindness. Window v. White, 163 N. E. 20, 79 S. E. 258.

68 See an article in 27 Harv. L. Rev. 644, by Professor D. O. McGovney.

69See supra, Sec. 30.

70 In Stensgaard v. Smith, 43 Minn. 11, 44 N. W. 669,19 Am. St. Rep. 205, it appeared that the defendant had promised the plaintiff the exclusive agency for three months of certain lateral contract such as is suggested is to be imported into the law, there seems logically little reason to exclude any offer from the same construction. If beginning performance is requested as the consideration of a collateral contract in the case of an offer for a unilateral contract, taking the offer under advisement is equally requested in an offer for a bilateral contract and should equally make it necessary for the offerer to hold his offer open for a reasonable time. This may well be desirable, but would better be reached as it has been in many European countries 71 by statute.

It is urged in support of the suggestion of a collateral contract, that the parties cannot contemplate that the offer may be revoked after part performance. Doubtless this is true. It is equally true that' parties do not generally understand that when an offer is given, which is expressed to be open for a stated time, that it may nevertheless be revoked before that time. In other words, parties do not altogether understand the law governing the formation of contracts, but mutual assent to rules of law is not necessary, though it is certainly true that a rule of law which is opposed to the understanding of business men is undesirable unless there are strong reasons of policy in its favor.72