Of 75 regions, some continental and others insular, the diversity as expressed in the number of Phanerogam genera is compared. As could be expected it is found that richness in both continental and insular regions is positively correlated with size and with proximity to source areas. In comparable continental and insular regions the former are always richer; increase in the number of genera with increasing size is stronger in the former. There are various factors disturbing the relations between size/diversity and isolation/diversity. The role of these factors, such as climate, age, topography and the like are discussed. It is shown that isolated islands are not always poor (New Caledonia, Fiji, Lord Howe I., Rapa, etc.). On the other hand the poverty of islands (e.g. New Zealand) need not be primarily due to the distance from a source area but may be caused by impoverishment of an originally rich flora. Once isolated, an island flora is much more subject to losses than a continental area where the losses in general may be readily replenished. It would be wrong therefore to conclude on the basis of poverty that an island has always been as much isolated as it is at present. Not only are isolated islands in general poorer in genera, there are also less genera per family than in a comparable continental area. This is shown to be caused by a preponderance of families represented by a single genus in the former.