IN Nature of November 11, S. L. Rabasa1 discusses the reduction in genetic variability which can take place in a population bred by artificial insemination. The problem he raises is not peculiar to populations bred in this way, but is a general one which must be faced in any population in which the parents of later generations are selected out of the whole number of adults available. The reduction in genetic variability resulting from using only a small number of animals as parents, and the loss of vigour which usually accompanies the unavoidable inbreeding, has to be weighed against the benefits resulting from the genetic superiority of the parents selected—a genetic superiority which can be increased only if the selected fraction is allowed to decrease. Artificial insemination makes it possible to have a smaller ratio of parents to non-parents but otherwise leaves the situation unchanged.