1-5. In judicial investigations it frequently becomes necessary for a jury to determine what is the reasonable value of services or property. In such cases expert and opinion evidence is admissible. There is an apparent, but no real analogy between this class of testimony and proof of reasonable time by like evidence in the present case. The distinction illustrates the *827reason underlying the rule. So far as reasonable value is concerned, opinion evidence is the only form of proof which can be made. At last, value is a matter of opinion. So many factors enter into its determination that it would be impossible for any witness to state them all to the jury, or, if he could, it would be impossible for them from such evidence alone to make the calculation. Even as to staple articles the selling price varies from day to day. It is affected by the activity or depression of business, the existence or want of competition and the infinite and complex causes operating under the law of supply and demand. For these reasons, and because no one can tell exactly wby anything will sell for a given amount, opinion evidence is admissible. In the nature of things it is the only class of evidence that can be given; and the Civil Code, § 5286, itself expressly recognizes it as proper to be received. The same principle would apply where no record of dates had been kept, or no timepiece had been used in measuring time. It would be impossible for the witness to state exactly how he judged of its lapse or duration. He could there- „ fore give his opinion as to how long he thought it was between two acts. So, too, there might be cases in which it would be proper for opinion evidence to be introduced as to what would be a reasonable time for the performance of a given act. But in such instances its. admissibility would grow out of the peculiar facts and result from the inability of the witness to 'give the data or to detail the circumstances on which he based his conclusion, and also from the impossibility of persons unfamiliar with the special business -making the calculation. But in the. present case there was no difficulty in the jury making the proper calculation as well as an expert, if they were furnished with the proper data. After considering all the circumstances and the conditions of the parties and.determining when the work of boxing the trees should begin, they could then easily determine how long it would take to complete that part of the enterprise. In like manner, it would be for them to determine from all the circumstances what was in contemplation of the parties, or what, under all the facts, would be a reasonable time within which to begin. This having been settled, given the quantity and character of the timber, the usual number of hands, and the capacity of the mill, it would be a mere matter of arithmetic to determine how long would be required to *828■cut the timber. To permit a witness to determine all these matters by answering what, in his opinion, was a reasonable time would have been to allow him to usurp the functions of the jury and to decide the very point in issue. What was a reasonable time must be determined by the facts as they existed in 1891. The condition of the parties and all of the circumstances were for consideration by the jury under instructions from the court. But it was not competent for a witness to give his opinion as to what was a reasonable time within which the trees should have been boxed and the timber cut. Such testimony involved not only calculations as to what could be done, but also a matter of law, as to What should be done. Mayor v. Wood, 114 Ga. 370.
6. Construing the contract, then, as it must be, according to its legal effect on the day it was executed and not according to subsequent changes, the court properly instructed the jury, that it included only timber which in 1891 was suitable for turpentine and sawmill purposes. The right to such trees, and to such trees only, passed at that date, as definitely as if each had been at that time marked and designated by the parties. If by growth others thereafter became suitable for turpentine or sawmill purposes, they did not thereby become subject to a contract which did not -originally include them within its terms. Martin v. Peddy, 120 Ga. 1079 (4); Warren v. Short, 119 N. C. 39 (3).
7. When- the jury, under the evidence and the charge of the -court, determined what would be a reasonable time for boxing and cutting the trees, they, in legal effect, could insert that date in the contract, and thereupon it would result that J. R. Allison or his assigns would have until the last day within which to exercise the rights granted. He or they could cut all the timber the first year, or all of it the last year. He or they during this period could cut a part of the trees on all or a portion of the tract. During the period found by the jury to be a reasonable time he or they could return and box or cut the rest. But neither he nor they could at the beginning of this period cut all of the timber suitable for sawmill purposes, and then return and cut such as by growth had become suitable for sawmill timber. Such, fairly construed, was the effect of the charge assigned as error. But if there was any doubt thereon, it was removed by a consideration of the succeeding sentence wherein the court instructed *829the jury that “ they would have had the right to have gone back,, provided it was in the reasonable time that was contemplated by the agreement of parties at the execution of the lease.”
8, 9. There was some evidence that the defendant cut dead timber which was lying on the ground; and complaint is made of the charge of the court, 'that-such timber did not pass by the conveyance of J891. • But the verdict in favor of the defendant shows that the jury found as a fact that a reasonable time had expired, and that the plaintiff no longer had any right to even the-growing timber. If this was true, the plaintiff, of course, had n& right to the dead timber, all having reverted to the grantor. So-that if the charge was incorrect, it was harmless. McRae v. Stillwell, 111 Ga. 65. Judgment affirmed.
All the-Justices concur.