The type of mating system we should use in any plan of livestock improvement requires very careful consideration. By reason of the particulate nature of inheritance and the behaviour of chromosomes in the cell divisions preceding the formation of eggs and sperm, the various mating systems differ in their influence upon the uniformity or otherwise of successive generations, the chances of securing improvement, the scope for control by selection, and finally, our ability to discriminate between the relative contributions of genetic and environmental variations to the population variance. Because of the genetic complexity many misconceptions have flourished about what we may expect with different mating systems, about the effects of inbreeding and the advantages and dangers of outbreeding. Different breeders have often secured different results with similar mating systems and the search for a rule of thumb guide has proved fruitless.

Prof. R. G. White prefaced his paper by referring to a letter which had been circulated among the members from Dr. Hugh C. McPhee of the United States Department of Agriculture at Washington. It had been hoped by the Executive Committee that some member of that Department might be in this country at the present time and be able to give an authoritative account of breeding in the United States but unfortunately this proved to be not possible. He would therefore try to lead up to some points brought out in Dr. McPhee's letter and give a short description of some centres at which research was being conducted. Before turning to those parts of the letter which he thought might form the basis of the greater part of their discussion he would venture to offer a few introductory remarks which he hoped would make it easier to appreciate the points brought out by Dr. McPhee.