What is the test for determining whether a defendant is responsible for the injury caused by a negligent shot?

The celebrated case of Summers v. Tice, supra, 33 Cal.2d 80, 199 P.2d 1, a unanimous opinion of this court, best exemplifies the rule. In Summers, the plaintiff was injured when two hunters negligently shot in his direction. It could not be determined which of them had fired the shot which [26 Cal.3d 599] actually caused the injury to the plaintiff's eye, but both defendants were nevertheless held jointly and severally liable for the whole of the damages. We reasoned that both were wrongdoers, both were negligent toward the plaintiff, and that it would be unfair to require plaintiff to isolate the defendant responsible, because if the one pointed out were to escape liability, the other might also, and the plaintiff-victim would be shorn of any remedy. In these circumstances, we held, the burden of proof shifted to the defendants, "each to absolve himself if he can." (Id., p. 86, 199 P.2d p. 4.) We stated that under these or similar circumstances a defendant is ordinarily in a "far better position" to offer evidence to determine whether he or another defendant caused the injury.