Arnold Gesell:
Inspired by Darwin’s work, Hall and his well-known student Arnold Gesell (1880–1961) devised theories of childhood and adolescence based on evolutionary ideas (genetic influences). These early leaders regarded development as a genetically determined process that unfolds automatically, much like a flower (Gesell, 1933; Hall, 1904).
Gesell was also among the first to make knowledge about child development meaningful to parents. If, as he believed, the timetable and pattern of development is the product of millions of years of evolution, then children are naturally knowledgeable about their needs. His child-rearing advice, in the tradition of Rousseau, recommended sensitivity to children’s cues. Along with Benjamin Spock’s Baby and Child Care, Gesell’s books became a central part of a rapidly expanding popular literature for parents.

From his lab at Yale University Gesell studied children of various ages and cataloged their behavior through the early developmental process. Based on those observations, he was able to identify NORMS of development; patterns of behavior that were typical and predictable at certain ages of development. This perspective is referred to as the "Normative-Descriptive Approach", since it utilizes norms of development to describe the growth process.
See Gesell video clip - HERE