"Everything has to evolve or else it perishes." How does this apply to Finny from A Separate Peace by John Knowles?

This quote "Everything has to evolve or it perishes" fits Phineas from A Separate Peace perfectly. Phineas is young, athletic, strong, and has a highly energetic personality. He looks forward to serving in the war effort because of these fabulous qualities. He has so much energy for life in him that he could accomplish anything he sets his mind to. All he has to do is pass high school and start his life's journey and he will soar like an eagle. Ironically, though, Phineas breaks his leg and is confined to a wheelchair for a few months at the beginning of his senior year. He eventually evolves to using a cane, which shows his progress in healing; but then he never gets a chance to progress past using it because he breaks his leg again!

Not only does Phineas decline physically, he also declines psychologically. After the first broken leg, Phineas realizes that he won't be able to go to the war and he becomes depressed. He doesn't admit this depression to anyone, of course, but it slowly leads him into denial about the war. Before he broke his leg for the first time, Phineas would talk about the news from the war; but after the broken leg, he outwardly declares that there is no World War II. His best friend Gene becomes very concerned about him at that point.

Referring to the Roaring Twenties, Phineas concocts a theory about the war being a lie and shares this with Gene as follows:

"Well what happened was that they didn't like that, the preachers and the old ladies and all the stuffed shirts. So then they tried Prohibition and everybody just got drunker, so then they really got desperate and arranged the Depression. That kept the people who were young in the thirties in their places. But they couldn't use that trick forever, so for us in the forties they've cooked up this war fake" (115).

Clearly, Phineas is on the decline here. He can't evolve like others in his class because he can't compete like he normally had. So he creates in his mind a world in which he doesn't feel as left out. It's as if he is using his own insecurities to create a reality in which he feels competent. The reality is, however, he is perishing, not evolving. Sadly, it all ends horribly when he breaks his leg for the second time and he physically perishes due to surgical complications.