Complex sentence: This tendency must favour the persistence of a new character of large volume suddenly appearing in a stock, and the observations of Mendel favour the view that the variations upon which natural selection has acted are not small but large and discontinuous.
Split into simpler sentences: This tendency must favour the persistence of a new character of large volume suddenly appearing in a stock. The observations of Mendel thus favoured the view that the variations upon which natural selection acts are not small but large and discontinuous.