Many in cognitive science have noted the importance of external visualizations for reasoning and learning, and have suggested that such visualizations play a role in complex reasoning contexts such as scientific investigation. However, what cognitive role diagrams play in scientific reasoning is unclear. I suggest that mechanistic diagrams function as search organizers in active research projects. Diagrams aid in scientific reasoning by being uniquely positioned to coordinate cognitive search through multiple search spaces, both within an individual and within a field. I examine this role using a number of published diagrams from mammalian chronobiology.