Theodosius Dobzhansky and Carl Epling
Carnegie Institution of Washington publicatino 554, Washington, D. C., March 31st, 1944
Contributions to the Genetics, Taxonomy, and Ecology of Drosophila pseudoobscura and its relatives.
“It is certain that if any kind of structural difference had been known between D. pseudoobscura and D. persimilis, they would have been classed as species from the start. Calling them races, and designating them by the letters A and B instead of by Latin names, was an attempt to appease conservative taxonomists who continue to adhere to the purely morphological concepts of species and race. Such a course is neither scientifically consistent nor practically sound. The species is the stage in the process of evolutionary divergence at which an array of populations once actually interbreeding or capable of interbreeding has become split into two or more reproductively isolated arrays. Species exist in nature regardless of whether we can or cannot distinguish them by their structural characters. There is no doubt that the great majority of animal and plant species differ structurally, and that they can be conveniently, and in most cases readily, recognized and delimited by their morphology alone. But it does not follow that any and all species are recognizable by their externally visible structures.”