This article examines how the genus category was perceived and conceived in zoology (with occasional references to botany), in reference to species on the one hand and to higher categories on the other hand. In systematic zoology and botany, animals and plants are classified and named according to their species, genera, and higher categories (family, order, etc.). Linguistic relationships between the words ‘genus’ and ‘general, generality’ might have played a role in some intuitive meaning of the genus. This article traces the evolution of the concept of genus as used in systematic zoology from antiquity to the present time, focusing on the contributions of Plato, Aristotle, Carl Linnaeus, Georges-Louis Leclerc de Buffon, Jean-Baptiste Lamarck, Georges Cuvier, and Charles Darwin. It also considers the introduction of a new, rank-free system called the PhyloCode to replace Linnaean ranking—and especially the genus level.