a series that had involved some notable lecturers in the past. It was back-dated so that Dobzhansky's lectures became the first in the series. The Columbia University Biological Series was also revived, and when Genetics and the Origin of Species was published in October 1937 it became the 11th volume in that series. Three main editions were published, in 1937, 1941 and 1951, each with significant changes. Dobzhansky considered Genetics of the Evolutionary Process (1970) a fourth edition, but one so much changed that it needed a new title. Contents of the first edition Genetics and the Origin of Species has two main points to make. The first is that speciation is a genuine problem that must be explained by the theory of evolution. In nature, there is not a single population of organisms separated from each other by small variations. Instead, the natural world is divided into species, each with its own limited range of variability. The second point is that all the variations can be explained by the principles of genetics. The 1937 edition was divided into nine chapters, whose contents are described below. Organic diversity The first chapter is a short outline of the main points of the book: Evolutionary theory must account for variations on the level of the individual and also that of populations and species. It must explain how reproductive isolation can occur. And the goal is to explain all this using genetic principles that can be verified in the laboratory. Gene mutation The second chapter argues that mutations occur frequently and are random in direction. Most are small in effect, but they affect all features of organisms and vary from beneficial to lethal. They are sufficient to provide the raw material for natural selection. Mutation as a basis for racial and specific differences In the third chapter, Dobzhansky demonstrates that the mutations that are observed in the laboratory also occur in nature, where they are acted on by natural selection. Thus, there is nothing intrinsically different about the process by which new species evolve. Chromosomal changes Dobzhansky continues the theme of continuity between the laboratory and nature in chapter 4, showing that variations in the structure, arrangement and number of chromosomes occur in both the laboratory and nature. He shows that chromosomal translocation, a rearrangement of parts in chromosomes, accounts for racial differences in Datura stramonium (jimson-weed). Chromosomal inversion, a reversal of a