What is the law of evidence in civil cases?

In Sopinka and Lederman, The Law of Evidence in Civil Cases (Toronto: Butterworths, 1974) at pp. 535-36 it is stated: In Blatch v. Archer Lord Mansfield stated: "It is certainly a maxim that all evidence is to be weighed according to the proof which it was in the power of one side to have produced, and in the power of the other to have contradicted." The application of this maxim has led to a well-recognized rule that the failure of a party or a witness to give evidence, which it was in the power of the party or witness to give and by which the facts might have been elucidated, justifies the court in drawing the inference that the evidence of the party or witness would have been unfavourable to the party to whom the failure was attributed.