Most biologists have assumed that play has indeed had some constructive purpose in species and individual evolution, and their advocacy is itself a much grander conceptualization than any modern attempts to rationalize human play simply as growth or socialization. The question is, are the empirical data of biology what ultimately fuel the notions of progress through play, or do our twentieth-century psychological and educational notions of progress through play bias us in favor of this interpretation of the data?

Sutton-Smith, The Ambiguity of Play p.19