The papers in this thesis represent a series of attempts — empirical and theoretical — to integrate developmental biology with population level studies of variation; to initiate a "developmental population biology" which would complement the well established fields of population ecology and population genetics. The introductory chapter traces the development of the conceptual ideas from the context of the maturation of a single research group. There follow three empirical chapters based on population studies of yellow cedar (Chamaecyparis nootkatensis). The first of these chapters examines the interdependency of progeny growth variation on parentage and stand structure and argues that parentage, developmental history and environmental contingencies can interact in complex ways to structure the variation observed in natural stands. The second chapter examines time related changes in patterns of variation for mainstem growth and needle initiation data of seedlings, and finds that the majority of the increase in variation with time results from differentiation among individual seedlings. The third chapter examines the nature of intra-individual variation in needle (from seedlings) and scale (from mature trees) data from the perspective of the concept of morphological integration, the amount and structure of covariation within an individual. The results of this chapter demonstrate that the nature of morphological integration changes during the course of development, and that variation in morphological integration — that is the pattern of variable relationships or covariance structure — distinguishes individuals. The final chapter is more theoretically oriented, and demonstrates how the patterns of increasing variation with time, and changing covariation with development (Chapters 2, 3) may be unified and explained in the context of developmental trajectories, where such trajectories represent the development of the form of individual organs through time in terms of point trajectories through a multivariate space. The nature of such developmental trajectories is ultimately a manifestation of cell division and elongation in various planes, resulting in the external form of the organs. Three increasingly complex graphical models of developmental trajectories are presented and it is argued that when developmental trajectories diverge from each other in a nonlinear manner, changes can occur in both correlation and covariance structures, coincident with changes in size. The relation between developmental trajectories and the production of variation within populations is further elaborated from the context of dynamical systems theory. / Science, Faculty of / Botany, Department of / Graduate