In Alabama Power Co. v. Berry, 254 Ala. 228, 48 So. 2d 231 (1950) the Court stated: "The function of the doctrine is to supply a fact which must have existed in the causal chain stretching from the act or omission of the defendant to the injury suffered by the plaintiff, but which the plaintiff, because of circumstances surrounding the causal chain, cannot know and cannot prove to have actually existed. The missing fact is that the defendant was negligent. The rationale of the theory, in part, is that the defendant in charge of the instrumentality which caused the injury is possessed of superior knowledge and by reason thereof is better advantaged than the plaintiff to know the true cause and therefore, negligence is presumed and the burden is upon the defendant to adduce proof to overcome the presumption." 254 Ala. at 236, 48 So. 2d at 238.