 Much of population genetics relied on an abstract concept of the gene as a unit inferred through character differences, represented in a symbolism, and used in the mathematical reconstruction or prediction of patterns of variation in theoretical, experimental, or natural breeding populations. In the early years of genetics, there was no agreement regarding the possibility of identifying units of the hereditary material and characterizing their nature and function. Considerations in this direction did not differ much from the speculative theories of the nineteenth century and were indeed mainly informed by them. In 1909 Johannsen coined the term “gene”, derived from de Vries’s term “pangene”, to disentangle the concept of the hereditary unit from any specific theory: