=278. Significance.= Endemism is readily recognized by methods of distributional statistics, applied to areas limited by natural barriers to migration or ecesis. For political areas, it has no significance whatever, unless the boundaries of these coincide with barriers. It determines in the first degree the validity of regions, though the latter are often recognized also by the presence of barriers and by the character of the vegetation. Endemism may occur in areas of vegetation of any rank from a formation to a zone. When the term is not qualified, however, it should be used of species with reference to formations alone. Comparisons to be of value, however, can be instituted only between areas of the same order, i. e., between two or more formations, two or more regions, provinces, etc. In the same way, taxonomic groups of the same rank should be used in such comparisons, i. e., species should be contrasted with species, genera with genera, and families with families, except when it is desired to obtain some measure of the age of the vegetation by the differentiation of the endemic phyla within it. There will be seen to exist a fundamental correspondence between the rank of the floral division and the taxonomic group, though the apparent exceptions to this are still too numerous to warrant its expression in a general law. As a rule, however, formations most frequently show endemic habitat forms and species, more rarely endemic genera; regions and provinces commonly exhibit endemic species and genera, rarely endemic families; while zones and hemispheres contain endemic orders as well as families. This correspondence is readily seen to depend primarily upon the fact that increased differentiation in the taxonomic sense is a concomitant of the increased invasion of endemic species, measured in terms of distance and difference in habitat.