According to Ernst Mayr, a new species forms when an existing species splits. A similar idea had been suggested in the 19th century by Moritz Wagner. Dobzhansky described the role of reproductive isolation in the formation of new species. Once a species lives in two different areas, the geographical isolation makes breeding between the groups reduce or stop. Each group develops features which make breeding between them work less well. Eventually, each group becomes a 'good' biological species, because the two species do not reproduce with each other even when they are together.