IN any trearon modern genetics, H. J. Muller figures as leman who discovered the action of X-rays Oromosomes and genes. It is this association wbgcji'at once comes to the mind of the biologist ondefc$j.mg of the award to him of the Nobel Prize for ftiysiology and Medicine for 1946. Yet this spectacular and in a way crowning achievement of his scientific career, when seen in the perspective of his whole work, is only one step along a road which was planned with brilliant foresight and imagination, and followed with critical and untiring accuracy. In 1927, when Mu'ller at the Genetical Congress in Berlin first produced definite proof that X-rays cause mutations, similar attempts, although without clear success, had already been made by a number of workers, and actually in the following year Stadler and others announced positive results of independent X-ray experiments with plants. Thus it was not the bare discovery of the metagenic action of X-rays which revolutionized genetics, but the manner in which Muller's previous work had paved the way for the use of it, and the genius with which he exploited it. First, in co-operation with T.H.Morgan in Columbia, later in the University of Texas, he had with great ingenuity used the fruitfly Drosophila to develop strains and methods, such as the CIB strain and balanced lethals, which formed and still form the basis for accurate tests of mutability. These methods, which already had borne fruit in studies of spontaneous mutability and its dependence on temperature, carried out by Muller alone and in co-operation with Altenburg, could now be put into the service of the new powerful agency for producing mutations. With their aid, progress in the new field was amazingly rapid. During the twenty years since its beginning, radiation genetics has proved a means of approach to a great number of fundamental problems of genetics: types of mutation, chromosome mechanics, gene action, position effect, size of gene, nature of mutation, to name only a few of them; and a very large share of the subsequent work has been due to Muller himself, or has at least been inspired or guided by him.