What is the test for a jury to award damages where damages are in part due in part to a defendant’s negligence?

Accordingly where damages are due in part to causes for which a defendant is responsible and in part to causes for which he is not responsible, the jury may not arbitrarily separate causes and damages without sufficient evidence so as to apportion and award damages for the cause for which the defendant is responsible. This will be an important matter when drafting the specific questions required on the retrial. We leave it to the sound discretion of the trial judge to formulate the required questions after hearing the evidence and submissions of counsel. We do however observe that his problem has arisen from time to time in nonjury cases. In Pryor v. Bains and Johal (1986), 69 B.C.L.R. 395 (B.C.C.A.), the principle to be applied in such cases was succinctly summarized in the headnote as follows: “Where a plaintiff claiming damages for personal injuries suffers from a pre-existing condition which was manifest and disabling before the injury inflicted by the defendant, the appropriate course is to determine the damages which would have been awarded had the plaintiff’s present condition resulted from a single cause for which is defendant was liable. The award should then consist of a portion of this amount equivalent to the portion of the plaintiff’s disability which can fairly be attributed to the defendant’s negligence. Cases in which a pre-existing condition is already manifest and disabling must be distinguished from the ‘thin skull’ cases where the weakness or latent susceptibility of the victim is quiescent but is activated into being as a result of the tortious conduct of another.”