520 MR. G. BENTHAM ON COMPOSITE. 
blended together that, with the data now at my eommand, I have 
been unable to separate them; and both present some general 
features which may justify the considering the two regions, for the 
present purpose, as one. 
The United-States region presents in its endemic, or almost 
endemic, genera a great contrast to the Mexican; the strictly 
endemie genera, indeed, which do not cross into Mexico or Cali- 
fornia are but very few ; but above thirty are almost confined to the 
region; and if we include in them the Euaster section of Aster, 
Solidago, and Helianthus, which, with Rudbeckia, Liatris, Sil- 
phium, Helenium, &c., are so characteristic of the region, we have 
an average of twelve or thirteen species, instead of about four, to a 
genus. The averages are brought much nearer together if we 
take into account the representatives of the genera belonging 
chiefly to other regions ; for whilst the Mexican region has a very 
large number of the southern genera Vernonia, Eupatorium, &c., 
the United States have but few species either of specially Mexican 
or southern genera or of those which belong to the northern 
regions of the Old World. The general average of species to a 
genus is thus brought to a little above six (774 species in 122 
genera) in the United-States region, against a little over five (1330 
species in 246 genera) in the Mexican. This remarkable deve- 
lopment ofthe endemic genera in the United States, compared 
with the paucity of their species in the Mexican region, may be 
taken as evidence of the prosperity of progressive races in the 
former, whilst the Mexiean region affords greater protection for 
the preservation of expiring races. 
There are, however, two or three of the United-States mono- 
types as remarkable in their isolation or distant connexions as any 
ofthe Mexican genera. Stokesia and Sclerolepis have no imme- 
diate affinities, the former being intermediate, as it were, between 
the otherwise distant tribes of Vernoniacez and Cichoriacez ; aud 
Sclerolepis, although technically placed among Eupatoriacc®, 
differs in habit and foliage from all other Composite. Chrys0- 
gonum is also remarkable from its connexion with the tropical- 
Asiatic Moonia, so close as to force us to unite them in one genus. 
The other endemic or prevailing genera belong to American tribes 
subtribes or genera, but with a greater proportion than in any 
other American region of the Old- World Anthemideze Cynaroidez 
and Cichoriacee ; whilst the South-American Mutisiacex are 
only represented by a single widely spread species, and the Old- 
