There is ample evidence (Pryor 1951,1957; Pryor, Chattaway, and Kloot 1956; Jackson 1958) that hybrids between species of Eucalyptus are of common occurrence in natural situations. Most of these naturally arising hybrids have proved to be quite fertile and it is therefore clear that such ready hybridization must tend to break down the distinctions between species and lead to a continuity of variation throughout the genus unless some contrary mechanism, helping to maintain specific identity, is in operation.