In times of war, people and societies change to adapt to the lifestyle war brings with it. In John Knowles" A Separate Peace the characters must adapt to the lifestyle brought forth by World War II. Some make it through these times better than others and some just don't make it at all. This brings me to my reasoning for the theme of this novel; "survival of the fittest". .
From the beginning of this novel it is made abundantly clear that Phineas and Gene are the fittest. In chapter one Phineas is described as ".the best athlete in the school" with "a strong neck in and uninterrupted, unemphatic unity of strength."(8) Clearly Phineas is the fittest physically. His physical build and superior athletic ability give him that advantage over everyone else enabling him to survive. Then there is Gene, who is also a good athlete but is no Finny. Gene couldn't be "just good but exceptional" in his academics (46). He has to stand out for his achievements. His just being good at school would not make him even with Phineas. Gene wants "to be head of the class, valedictorian."(43) Gene sees it that Finny is the best athlete in school and he is his best friend so he has to be best at something too. You don't stand out for just being good at something you have to be exceptional to survive. Another character of this novel, Lepper Leppellier, also comes to the realization of the survival of the fittest in this quote: "I"m almost glad this war came along. It is like a test, isn't it, and only the things and people who've been evolving in the right way survive."(117) And later in the novel we find out that he was definitely not the fittest, he went crazy. He did realize however, Gene to fit the characteristics he was referring to in the quote above. Well, with Phineas and his broken leg, Gene is not only the smartest but also the best athlete.
Indefinitely, Lepper realizes the "survival of the fittest" reality yet he is not the "fittest" due to the fact that the war made him crazy.