In the fictional Marvel Universe, secondary mutation is a global phenomenon among mutants that seems inexplicable, with many previously established mutant characters suddenly developing new or vastly expanded powers. Henry McCoy hypothesizes that, since the mutant population was entering the millions at that time, mutant physiology was responding to the need for survival. Thus in rare occasions a mutant's powers change or become even greater, and in rare cases they can develop a new, often unrelated ability. However, the criteria for secondary mutation is ill-defined at best, leaving many fans confused as to their favorite character's status or powers. This use of secondary mutations can sometimes be misused as a deus ex machina plot device; if the writer can't think of a way for a character to get out of a situation, they can simply develop a spontaneous new power to handle the problem.