Benjamin S. Orlove has noted that the development of ecological anthropology has occurred in stages. "Each stage is a reaction to the previous one rather than merely an addition to it". The first stage concerns the work of Julian Steward and Leslie White, the second stage is titled 'neofunctionalism' and/or 'neoevolutionism', and the third stage is termed 'processual ecological anthropology'. During the first stage, two different models were developed by both White and Steward. "The distinction is not as rigid as some critics have made it out to be, White's models of cultural evolution were unilinear and monocausal, whereas Steward admitted a number of different lines of cultural development and a number of different causal factors. During the second stage, it was noted that the later group agreed with Steward and White, while the other disagreed. 'Neoevolutionists' borrowed from the work of Charles Darwin. The general approach suggested that "evolution is progressive and leads towards new and better forms in succeeding periods". 'Neofunctionalists' "see the social organization and culture of specific populations as functional adaptations which permit the populations to exploit their environments successfully without exceeding their carrying capacity". 'Processual ecological anthropology' is noted to be new. Studies based on this approach "seek to overcome the split in the second stage of ecological anthropology between excessively short and long time scales". The approach more specifically, examines "shifts and changes in individual and group activities, and they focus on the mechanism by which behavior and external constraints influence each other".