Genetics, as we saw in the Introduction, grew out o f the practical problem o f the improvement o f agricultural crops and stock. The development o f the subject which followed Mendel’s discoveries has not, however, been primarily concerned with practice, but has been towards the formulation o f a comprehensive theory and the elucidation o f the fundamental biological problems which were raised. This scientific advance has only fairly recently begun to have much effect on breeding practice, and we are still a long way from being able to apply the whole o f the theoretical knowledge at our disposal. In the peculiar economic situation in which man has found himself during the last few decades, production, even with little help from science, has been so far ahead o f consumption that the application o f genetics to this field has, in most countries, not been investigated whole-heartedly and on a large scale. The results which have been obtained are, however, already quite considerable, and we can envisage the possibility o f quite startling changes in the world’s agricultural economy, particularly in connection with crop plants.