INVASION 229 
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. €., be- 
tween 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 com- 
parisons, 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 appar- 
ent 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 prov- 
inces 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 in- 
creased invasion of endemic species, measured in terms of distance and dif- 
ference 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 transition area between regions would seem to point to the opposite con- 
clusion, 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 investi- 
gation of their comparative differentiation in connection with their migration 
in all directions from the vegetation center 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 phytogeog- 
raphy is the division of the earth’s vegetation into natural areas. It will 
be at once evident that most plants can not properly be called endemic until 
the natural regions in which they are found have been accurately defined, 
a work which has barely begun. In the much simpler matter of distribution, 
upon which the accuracy of statistical methods depends directly, there are 
few regions sufficiently well known at the present time to yield anything 
like permanent results. 
