In A Separate Peace, Leper concludes only people who've been "evolving" the right way survive war, a test. How does this apply to Gene and Leper?

The effects of the war on Gene and Leper seem to support Leper's theory. After enlisting, Leper is destroyed by the war--or more specifically, by the demands placed upon him by military service. A gentle, introspective, and isolated individual, Leper cannot stand up to the authoritarian and regimented environment in which he finds himself. Leper had not "evolved in the right way" because he had never learned to live and function in the society of the real world. He had found sanctuary instead in a world of his own creation, whether it was playing with his snails or setting out alone to hike the snowy hills around Devon. Leper lived life just as he had played blitzball, on the periphery. When he enlisted in the army under the illusion that war could be a clean, beautiful skiing experience, he was instead drawn into the center of social chaos. He could not "evolve" fast enough to adapt and survive; he breaks emotionally and psychologically.

For Gene, however, the war becomes almost an anticlimax after the fear and turmoil of his experiences at Devon. Gene struggled with forces within and without, and in the process became stronger and more self-aware. Through Finny, Gene eventually learned how to deal with life when it became overwhelming:

During the time I was with him, Phineas created an atmosphere in which I continued now to live, a way of sizing up the world with erratic and entirely personal reservations, letting its rocklike facts sift through and be accepted only a little at a time, only as much as he could assimilate without a sense of chaos and loss.

At the novel's conclusion, Gene explains himself in relation to the war, after he had experienced it:

I never killed anybody and I never developed an intense level of hatred for the enemy. Because my war ended before I ever put on a uniform; I was on active duty all my time at school. I killed my enemy there.

According to Leper's theory, Gene had "evolved" sufficiently to survive psychologically when he emerged from one war to participate in another.