During the 20th century scientists have made genuine progress in explaining and modifying usefully the processes of life. The main leap forward was, of course, the rediscovery of Mendelian genetics at the beginning of the century. Since the 1930s this theory has been put to use in an ever more efficient and systematic way, particularly in breeding domestic animals with desired traits. Thus the animal breeder can now plan how he wants future generations of domestic animals to be, knowing that the plans will work.