What is the test for determining whether a consequence is natural and probable?

deciding whether a consequence is natural and probable, consider all of the circumstances established by the evidence. [] If you decide that the defendant committed murder, it is murder of the second degree, unless the People have proved beyond a reasonable doubt that it is murder of the first degree as defined in CALCRIM No. 521." The jury was thereafter instructed with CALCRIM No. 521 on first degree murder under the theory the killing was willful, deliberate, and premeditated. The jury found the murder was first degree under this theory, thereby finding express malice. (People v. Cox (1888) 76 Cal. 281, 285 ["if the evidence manifestly proves beyond a doubt that the 'killing' was premeditated, etc., the 'express malice' is proved"].)