David Hull has contributed a number of central and influential views on the topic of conceptual change in science. Our aim in this chapter is to follow up and elaborate on some issues he has delimited, in the context of a case study of the evolution of diagrams of Weismannism. Hull has been a proponent of an evolutionary analysis of change in science, but he also argues that application of this approach demands more than many of its advocates seem to suppose: in order to use evolutionary concepts to analyze scientific change, not only do we need to understand what biologists mean by evolution, but we must use their conceptual tools correctly. This means that a substantial amount of conceptual analysis of biological concepts and methods must go into an evolutionary theory of conceptual change, and that the metaphorical use of evolutionary language must not be considered a substitute for analysis.