What is the case law on admitting evidence in the context of evidence at trial?

"The genius of the more thoughtful and judicious decisions of the modern day is to admit into evidence any rational proof that may have a tendency to illuminate the transaction on trial or materially aid in determining the issue but subject, of course, to the discretion of the trial court in excluding extraneous matter. (See Evidence, 18 Cal.Jur.2d 116, p. 555.)" (Odell v. Frueh (1956) 146 Cal.App.2d 504, 511-512, 304 P.2d 45, 51.)

We turn to consider defendant's second contention: