During the postwar period, the concept of instinct came to encapsulate the debate around the importance of nature versus nurture. The fact that animals show highly organized behavior early in development suggested the presence of an underlying fixity where behavior was “inbuilt” into an animal’s biology despite an individual’s experiences. This placed a discrete and exhaustive line between the innate and acquired that became a foundation for the European-dominated field of ethology. Across the Atlantic, a group of comparative psychologists led by the American Museum of Natural History’s T. C. Schneirla contested this approach, proposing that the study of animal behavior should avoid abstract dichotomies with a renewed focus on developmental processes. While Schneirla’s theoretical and empirical work shaped the modern study of animal behavior, his legacy requires revisiting in an era where the nature versus nurture debate is regaining prominence. In this article, I revisit Schneirla’s approach to behavior with a focus on his paper “Interrelationships of the ‘Innate’ and the ‘Acquired’ in Instinctive Behavior” (published in M. Autuori et al. (1956) L’instinct dans le comportement des animaux et de l’homme; Masson, Paris, pp. 387–452) for the journal’s “Classics in Biological Theory” collection; the paper is available as supplementary material in the online version of this article. A companion article (this issue; G. M. Kohn (2024) “A Discussion on Instinct, Paris, 1954”) presents the commentary that was published with it.