 In 1940, he was the first to describe and define genetic polymorphism, and to predict that human blood group polymorphisms might be maintained in the population by providing some protection against disease.His 1949 book "Mendelism and Evolution" helped to persuade Dobzhansky to change the emphasis in the third edition of his famous textbook "Genetics and the Origin of Species" from drift to selection.Ivan Schmalhausen developed the theory of stabilizing selection, the idea that selection can preserve a trait at some value, publishing a paper in Russian titled "Stabilizing selection and its place among factors of evolution" in 1941 and a monograph "Factors of Evolution