This chapter is concerned with the history of scientific thought about cells—changes in how the cell has been defined, what have been taken to be its essential properties, and how it stands in relation to larger organisms as a whole (i.e., considerations of anatomy, physiology, and development). In addition, this chapter considers how the cell theory merged with the theory of evolution, following the publication of Darwin's On the Origin of Species in 1859. It then became relevant to ask how cells themselves have evolved, both as individual living units and as components of larger social aggregates. The discussion is limited roughly to the middle third of the nineteenth century (from 1838 to 1872 or so), ranging from the establishment of "the cell theory" of Schleiden and Schwann to Ernst Haeckel's speculations about the relationship between ontogeny and phylogeny, including an examination of Darwin's thoughts about the cell theory as expressed in his theory of pangenesis of the late 1860s.