delivered the opinion of the court.
In the statute referred to it is provided that: “ * * Generally, all owners, contractors or subcontractors and other persons having charge of, or responsible for, any work involving a risk or danger to the employees or the public, shall use every device, care and precaution which it is practicable to use for the protection and safety of life and limb, limited only by the necessity for preserving the efficiency of the structure, machine or other apparatus or device, and without regard to the additional cost of suitable material or safety appliance and devices. ”
*5191. The statute is analogous to what is known as the factory act: L. O. L., § 5040 et seq. An absolute duty is imputed to the employer for the violation of which he is penally, as well as civilly, liable. Under such circumstances, the servant does not assume the risk of injury, as decided in Hill v. Saugested, 53 Or. 178, 185 (98 Pac. 524, 22 L. R. A. (N. S.) 634). Within the rule laid down by Mr. Justice Bean in that well-considered case, and followed by Mr. Chief Justice McBride in Love v. Chambers Lumber Co., 64 Or. 129 (129 Pac. 492), the court was not in error in the case at bar in refusing three instructions on the subject, of assumed risk.
2. There was some testimony before the jury to the effect that the injuries received by the plaintiff in the fall aggravated an old complaint of appendicitis from which he had previously suffered. The defendant asked the court to instruct the jury that they could not take the aggravation of the former complaint into consideration, because the same had not been pleaded in the complaint, but the court refused to give the instruction, and failed to give anything to enlighten the jury on that point. While, as stated in Guild v. Portland Ry., L. & P. Co., 64 Or. 570 (131 Pac. 310, 312), “the negligent injury of one who is weak and incapacitated in person is as culpable as any other ill usage,” still, if the plaintiff would recover for an aggravation of a former persistent injury, he must plead the same: Maynard v. Oregon R. R. Co., 46 Or. 15 (78 Pac. 983, 68 L. R. A. 477). Under a pleading like the present complaint, all mere aggravation of former injury must be laid aside. The plaintiff can, of course, recover for the direct effect of the injury arising from the negligence of the defendant, notwithstanding, as an incident thereto, the former complaint may be aggravated; but nothing can be recovered for the aggravation itself, unless the same is averred.
*5203. The evidence on this subject could not be stricken out because it is intimately connected with the direct effect of the injury complained' of; but it was the duty of the court to caution the jury against allowing anything for aggravation, the narration of which incidentally crept into the testimony.
4. The defendant requested, and the court neglected to give, instructions to the effect that the plaintiff must establish the negligence of the defendant by the preponderance of the evidence; that negligence cannot be presumed from the mere happening of an accident; that the defendant is presumed to be innocent of the negligence; that the mere happening of the injury or accident is not alone sufficient to charge the defendant, but that there must be some evidence attributing it to the negligence of the defendant. It is provided in Section 868, L. O. L., that “the jury, subject to the control of the court, in the cases specified in this code, are the judges of the effect or value of evidence addressed to them,*except when it is thereby declared to be conclusive. They are, however, to be instructed by the court on all proper occasions. * * 5. That in civil cases the affirmative of the issue shall be proved, and when the evidence is contradictory, the finding shall be according to the preponderance of evidence. * * ” It is mandatory, therefore, upon the court in a case where the allegations of the complaint are traversed, to instruct the jury that the plaintiff must prove his case by the preponderance of the testimony. Again, if the complaint is true, the defendant is guilty of a tort or wrong. It is laid down as a presumption by Section 799, L. O. L., “that a person is innocent of crime or wrong.” This presumption is a piece of evidence that the defendant was entitled to have submitted to the jury. Its refusal was error. It is, indeed, true that the statute under which this action was instituted greatly enlarges the duties of an em*521ployer, the violation of which constitutes culpable negligence in case an injury ensues; but the general rules for proving or combating a charge of such negligence have not been changed by the statute under consideration. The court was in error in not observing the directions of the code on the subjects indicated.
5. The injury complained of happened in the office of a wholesale drug firm. A foundryman, shown to be familiar with the use of ladders only in his establishment, was called as an expert, and was allowed to testify over the objection of the defendant to the effect that in his opinion the plaintiff, having the ladder only for that purpose, was not supplied with suitable appliances for making the alterations in the building of the defendant in the place where the accident occurred. As a rule, expert testimony and opinion evidence are to be considered only to illustrate and explain to the jury a complicated situation not ordinarily comprehended by men of common intelligence, and involving some difficult subject of art or science. Usually it is not sufficient alone to decide any question independent of other testimony about the facts involved. It is receivable for the ancillary purpose of making more comprehensible intricate facts detailed by other witnesses who testified directly to those facts.' Whether or not the ladder was a suitable appliance for the work in hand with which the plaintiff was charged was peculiarly a question for the jury, and the authorities are unanimous that no witness can invade the province of the jury and undertake to decide by his opinion the issue committed to the twelve men. Conceding, without deciding, that so simple an appliance as an ordinary ladder could be made the subject of expert testimony, and that a man shown to be familiar only with conditions in a foundry was competent to give an opinion on conditions in the office of a drug company, yet it was clearly *522error to permit Mm to give Ms opinion on the ultimate question to be decided.by the jury, namely, whether or not the ladder in question was a suitable appliance for the task at hand.
The judgment of the Circuit Court is reversed, and the cause remanded for further proceedings not inconsistent with this opinion. Reversed.
Mr. Chief Justice McBride, Mr. Justice Moore and Mr. Justice Ramsey concur.