"Organicism" often refers to the idea that ecosystems or communities are, or are like, organisms. Often implicit in early twentieth century, it has been theorized by Clements, relying on physiological and developmental concepts. I investigate the fate of this idea in major attempts of a theoretical synthesis of ecology in the first part of the twentieth century. I first consider Bioecology (1939), by Clements and Shelford, which elaborates clementsian organicism as a general framework for plant and animal ecology. Then I investigate the major animal ecology treatise of the Chicago school ecologists C. Allee, T. Park, O. Park, K. Schmidt and A. Emerson, Principles of animal ecology (1949). I show how they shifted organicism from physiology to evolution, synthesizing inspiration from both Clements and Sewall Wright, got their inspiration in evolutionary biology, and built a systematic correspondence between cells, organisms and communities. I claim that the focus on populations allowed them to apply Darwinian insights at the level of communities. Finally I argue that this theoretical synthesis fell apart in the next decade because of the rise of density-dependent accounts of population regulation.