Morss (1990) traces the naturalistic assumptions behind the traditional developmental paradigm back to the Romantic notions of social progress and human perfectibility which emerged in the nineteenth century. Theories about evolution proliferated at this time and the views which were taken up by influential figures like Herbert Spencer and G. Stanley Hall were in many ways pre-Darwinian. Early developmentalists were certainly more concerned with notions to do with recapitulation and evolutionary hierarchies than with natural selection. As Morss notes,

Perhaps the most fundamental assumption concerning an overall picture of individual development is that of progress. Derived from, or at least legitimated by biological sources, the notion that the individual gets better and better as time passes has been central to most developmental thinking.