It is second only to maize in exploiting the principles of heterosis and crossbreeding. By crossbreeding and selective breeding it tries to establish similarity to the aurochs. To prevent spawning, sterile oysters are now cultured by crossbreeding tetraploid and diploid oysters. Bulls have been used as sires for crossbreeding. Results of crossbreeding classic and woolly breeds of llama are unpredictable. Tonkinese are a domestic cat breed produced by crossbreeding between the Siamese and Burmese. Charles Darwin postulated that modern goats arose from crossbreeding markhor with wild goats. Ocicats came about from the accidental crossbreeding of Abyssinians and Siamese breeds. More recent observation, however, has documented significant crossbreeding and hybridization in the wild. Following the rising demand for high-quality beef, new breeds and selective crossbreeding have been developed. In cattle, there are systems of crossbreeding. Jack Russell traits often remain visible despite this crossbreeding. Eventually, out of crossbreeding Bulldogs and terriers, the English created the Staffordshire Bull Terrier. Charles Dalton and Robert Oulton began crossbreeding experiments in 1894. Their crossbreeding experiments in facilities called "farms" are unsuccessful.