442 MR. G. BENTHAM ON COMPOSITE. 
phieal isolation indieated a relationship which would not other- 
wise have been so clear. 
Bidens, a genus of about fifty species (nearly doubled by some 
botanists), although technically distinguished from Coreopsis by a 
somewhat trivial charaeter, the asperities of the awns of the 
achenes directed downwards instead of upwards, is nevertheless 
a natural genus ; and although geographically it may have as wide 
a range as Coreopsis, its distribution has a different character. The 
genus has two natural sections; one (Platycarpea) is so generally 
diffused over the temperate regions of the northern hemisphere 
that it would be difficult to determine whether its origin is Ame- 
rican or Europxo-Asiatic; and the representative species in the 
two regions differ but little from each other. "There are, however, 
two or three American species unrepresented in the Old World; 
and Cassini’s St.-Domingo genus Narvalina (a single species) 
may be considered as a divergence only from the Platycarpea, 
thus confirming some other evidences of the American ancestry of 
the group. One of our common species, B. tripartita, Linn., repre- 
sented in America by B. frondosa, Linn., and B. connata, Muehl., 
reappears in the southern mountain-ranges of Australia. The 
other section, Psilocarpea, is more tropical and essentially Ame- 
rican. Two species are indeed amongst the commonest weeds all 
over the warmer regions of the Old World; but that is a case 
similar to that of the Elephantopus, if, indeed, the presence of these 
species in some districts be not due to modern importations, won- 
derfully facilitated by the prehensile nature of the awns of the 
achenes. 
A Sandwich-Island Bidens, in its reduced pappus, shows an 
anomalous insular form, and may possibly be derived rather from 
the Coreopsides (Campylothecas) of the same islands. At any 
rate, this group shows the connexion of Coreopsis with Bidens, 
and is an example of divergence, with different combinations of 
characters in the isolated islands, from those which have become 
established in the general continental area of the genera. 
Various groups, further diverging from Coreopsis and Bidens, have 
arisen in various portions of the extended area of the genera :— 
in west tropical America, extending more or less from Bolivia to 
the Mexican region, Dahlia with four or five species and Cosmos 
with ten; in east tropical America Isostigma, five species; in 
subtropical America, north and south (Mexican and Bonarian 
regions), Thelesperma, four or five species; and in east tropical 
Asia and Australia, Glossogyne, five species. 
