This book reports a study which examined how the attainment of theoretical frameworks may create the conditions for and support subsequent learning of related material. Specifically, Vygotsky's proposal that learning only occurs in the Zone of Proximal Development was investigated. The relationship between the actual level of development, as reflected by an ability to do proportional reasoning, and potential development, measured by the ability to perform certain strategic procedural operations in the molecular biosciences which were underpinned by proportionality, was probed in a class of second year students registered at the University of the Witwatersrand, Johannesburg, South Africa. The study also questioned which factors might have augmented the development of proportional reasoning ability.