Evolutionary biologists have long sought to understand the relationship between microevolution (adaptation), which can be observed both in nature and in the laboratory, and macroevolution (speciation and the origin of the divisions of the taxonomic hierarchy above the species level, and the development of complex organs), which cannot be witnessed because it occurs over intervals that far exceed the human lifespan. The connection between these processes is also a major source of conflict between science and religious belief. Biologists often forget that Charles Darwin offered a way of resolving this issue, and his proposal is ripe for re-evaluation in the light of recent research.