  Notice that the reasoning here does not depend on identifying the material make-up, mode of action, or general function of the underlying gene. It depends only on the ideas that copies of the gene are distributed from generation to generation and that the difference in the gene (i.e., the difference between pr and +), whatever this difference is, causes the phenotypic difference. The idea that the gene is the difference maker needs to be qualified: differences in the gene cause phenotypic differences in particular genetic and environmental contexts. This idea is so crucial and so often overlooked that it merits articulation as a principle (Waters 1994):