Austin, on such classic story arcs as "Proteus", "Dark Phoenix Saga", and "Days of Future Past" would make them both fan favorites. Byrne insisted that the title keep its Canadian character, Wolverine, and contributed a series of story elements to justify Wolverine's presence which eventually made the character among the most popular in Marvel's publishing history. With issue #114, Byrne began co-plotting the series as well as penciling. Claremont recounted that "at that point in time John and I were, in a very real sense, true collaborators on the book. It was with very few exceptions, difficult, for me, anyway, to tell in the actual gestation of the book where one of us left off and the other began – because it involved one of us coming up with an idea and bouncing it off the other ..." The "Dark Phoenix Saga" in 1980 is one of the most notable stories in the title's history. Comics writers and historians Roy Thomas and Peter Sanderson observed that "'The Dark Phoenix Saga' is to Claremont and Byrne what the 'Galactus Trilogy' is to Stan Lee and Jack Kirby. It is a landmark in Marvel history, showcasing its creators' work at the height of their abilities." Byrne has repeatedly compared his working relationship with Claremont to Gilbert and Sullivan, and has said that they were "almost constantly at war over who the characters were." Byrne created the characters Alpha Flight, Proteus, and Kitty Pryde/Shadowcat during his run on The X-Men. A new Brotherhood of Evil Mutants, led by Mystique, was introduced in the "Days of Future Past" storyline (#141–142, Jan.-Feb. 1981) in which a time-travelling Kitty Pryde tried to avert a dystopian future caused by the Brotherhood assassinating Presidential candidate Senator Robert Kelly. Byrne plotted the story wanting to depict the Sentinels as a genuine threat to the existence of the mutant race. Byrne left The X-Men with #143 (March 1981). During his tenure on the series, The X-Men was promoted from a bimonthly to a monthly publication schedule as sales steadily increased—a trend that continued long after Byrne left. In the late 1970s, while serving as the regular penciller of X-Men, Byrne began penciling another superhero team title, The Avengers. Working for the most part with writer David Michelinie, he drew issues #164–166 and 181–191. Byrne and Michelinie co-created Scott Lang in Avengers #181 (March 1979). Byrne's nine-issue run of Captain America,