The Catholic Church holds no official position on the theory of creation or evolution, leaving the specifics of either theistic evolution or literal creationism to the individual within certain parameters established by the Church. According to the Catechism of the Catholic Church, any believer may accept either literal or special creation within the period of an actual six-day, twenty-four-hour period, or they may accept the belief that the earth evolved over time under the guidance of God. Catholicism holds that God initiated and continued the process of his creation, that Adam and Eve were real people, and that all humans, whether specially created or evolved, have and have always had specially created souls for each individual. Early contributions to biology were made by Catholic scientists such as the Augustinian friar Gregor Mendel. Since the publication of Charles Darwin's On the Origin of Species in 1859, the attitude of the Catholic Church on the theory of evolution has slowly been refined. For nearly a century, the papacy offered no authoritative pronouncement on Darwin's theories. In the 1950 encyclical Humani generis, Pope Pius XII confirmed that there is no intrinsic conflict between Christianity and the theory of evolution, provided that Christians believe that God created all things and that the individual soul is a direct creation by God and not the product of purely material forces. , the Church supports theistic evolution, also known as evolutionary creation, although Catholics are free not to believe in any part of evolutionary theory. Catholic schools in the United States and other countries teach evolution as part of their science curriculum. They teach the fact that evolution occurs and the modern evolutionary synthesis, which is the scientific theory that explains how evolution proceeds. Early contributions to evolutionary theory Catholics' contributions to the development of evolutionary theory included those of the Augustinian monk Gregor Mendel (1822-1884). Mendel entered the Brno Augustinian monastery in 1843, but also trained as a scientist at the Olmutz Philosophical Institute and at the University of Vienna. The Brno monastery was a centre of scholarship, with an extensive library and tradition of scientific research. At the monastery, Mendel discovered the basis of genetics following long study of the inherited characteristics of pea plants, although his paper Experiments on Plant Hybridization, published in 1866, remained largely overlooked until the start of the next century. He developed mathematical formulae to explain the occurrence, and confirmed