However, around 1900 a reliable trap nest was invented (making it possible to register which egg belongs to which hen). Besides, the Mendelian principles of hereditary were recognised; poultry were a major species to test these principles – in this way it became possible to breed for production! Early in the 20th century, over all continents existing breeds were selected for egg and/or meat production. Around 1940 it was detected in the USA that the crossing of breeds offered an advantage concerning production traits.