ENDEMISM 67 



more regions, provinces, etc. In the same way, taxonomic 

 groups of the same rank should be used in such compari- 

 sons, 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 numer- 

 ous 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 ;«ones and hemispheres don- 

 tain endemic orders as well as families. This correspon- 

 dence is readily seen to depend primarily upon the fact that 

 increased differentiation in the taxonomic sense is a con- 

 comitant of the increased invasion of endemic species, 

 measured in terms of distance and difference in habitat. 

 It is too early to decide satisfactorily whether it is proper 

 to speak of formations as endemic. At first thought, it 

 would seem that all formations, with the exception of 

 ruderal ones, were endemic, but a study of almost any trans- 

 ition area between regions would seem to point to the 

 opposite conclusion, viz. that no formations are properly 

 endemic. It is equally impossible at present to distinguish 

 different types of endemics, such as relictae, etc., as any 

 such classification must await the elaboration of a method 

 for determining the phylogeny of a natural group of species 

 by an investigation of their comparative differentiation iu 

 connection with their migration in all directions from the 

 vegetation centre into new habitats. In short, it will not be 

 possible to make a thorough study of endemism and to 

 postulate its laws until modern methods of research ^have 

 been extended to a much larger portion of the vegetation of 

 the globe. The final task of phytogeography is the division 

 of the earth's vegetation into natural areas. It will be at 

 once evident that most plants cannot properly be called 



