Rippleton Holabird plans to leave McGurk Institute and help Tubbs guide the League of Cultural Agencies. He offers Martin the position of Assistant Director, soon to become full Director. Joyce is elated with the prospect. They talk the matter over at length, but before morning he moves out of the house and into a cheap hotel, refusing to answer Joyce's telephone call. Next day, he catches a train for Vermont and joins Terry "for keeps."
The harsh life of Birdies' Rest is in contrast to the luxury to which Martin has been accustomed. He becomes adjusted to it, however, and is soon engrossed in experimentation with quinine derivatives. His work draws ahead of Terry's. His mathematics and physical chemistry are now as sound as and his imagination more swift than that of his companion. Terry plans to bring in a few more scientists, making a total of eight.
Joyce arrives one day, ready to make peace and to build a house across the lake. When he objects to her bringing people to intrude on his privacy, she says that he is becoming a fanatic and regretfully leaves him. She will probably divorce him and marry Latham Ireland.
Dr. Aaron Sholtheis becomes Director of McGurk Institute; Dr. Angus Duer is Head of Duer Clinic, Chicago; Bert Tozer is still attending midweek prayer meeting in Wheatsylvania, North Dakota; and Max Gottlieb sits "unmoving and alone" in his little room above the city street. Martin and Terry are discussing their new quinine experiments as the novel closes.
The ending of Arrowsmith is considered less satisfactory than earlier portions of the book. Yet Lewis disposed of practically every important character mentioned, from the members of Digamma Pi to the doctors and scientists of the New York and Chicago clinics. Martin's refusal to compromise his ideals for the sake of wealth and popularity is consistent throughout the novel, giving him the rare attributes of the true scientist.