The properties of continuous variation are basic to the theory of evolution and to the practice of plant and animal improvement. Yet the genetical study of continuous variation has lagged far behind that of discontinuous variation. The reason for this situation is basically methodological. Mendel gave us not merely his principles of heredity, but also a method of experiment by which these principles could be tested over a wider range ofliving species, and extended into the elaborate genetical theory of today. The power of this tool is well attested by the speed with which genetics has grown. In less than fifty years, it has not only developed a theoretical structure which is unique in the biological sciences, but has established a union with nuclear cytology so close that the two have become virtually a single science offering us a new approach to problems so diverse as those of evolution, development, disease, cellular chemistry and human welfare. Much of this progress would have been impossible and all would have been slower without the Mendelian method of recognizing and using unit differences in the genetic materials. These great achievements should not, however, blind us to the limitations inherent in the method itself. It depends for its success on the ability to assign the individuals to classes whose clear phenotypic distinctions reveal the underlying genetic differences.

In the second edition of Biometricai Genetics, which appeared in 1971, we set out to give a general account of the subject as it had developed up to that time. Such an account necessarily had to be comprehensive and reasonably detailed. Although it could be, and indeed has been, used by those who were making an acquaintance with this branch of genetics for the first time, it went beyond their needs. We have been encouraged therefore to write an introduction to the genetical analysis of continuous variation aimed primarily at senior undergraduate and postgraduate students, and concentrating on basic considerations, basic principles and basic techniques. This has meant, of course, omitting all reference to some phenomena of more restricted interest, notably sex-linkage, ma ternal effects, haploidy and polyploidy. It has meant, too, that even with some phenomena which have been included, like interactions, linkage and effective factors, the discussions cannot go into full detail. Anyone who is interested, however, can find further information in Biometricai Genetics, to which detailed references have been given where it ap peared that these would be helpful. The order of presentation has been changed with the aim of making it easier for beginners.