What is the test for determining whether a tortfeasor has been found to have been responsible for a second source or cause of damages?

128 In Pryor v. Bains, supra, Carruthers, J.A. said at p. 397: These two sources or causes of damages can be dealt with, either as a case of aggravated damages or as a "thin skull" case, depending on which of two factual circumstances are found to exist. In a case where a second source or cause of damages is found to aggravate an existing and active first source or cause, that is a case of aggravated damages and there may be an apportionment of damages as between the two sources or causes. On the other hand, in a case where a second source or cause of damages triggers the first source or cause which has been found immediately prior to the injury to be merely a latent weakness or susceptibility and not an active source or cause, that is a "thin skull" case and there can be no apportionment as between the two sources or causes and full damages must be awarded against the tortfeasor creating the second source or cause of damages which triggered the latent first source or cause.