"The statement of a living man is not to be disbelieved because there is no corroboration, although in the necessary absence through death of one of the parties to the transaction it is natural that, in considering the statement of the survivor, we should look for corroboration in support of it; but, if the evidence given by the living man brings conviction to the tribunal which has to try the question, then there is no rule of law which prevents that conviction being acted upon."