216 James Smalt. 
and the habit. The Baccharidinae can, therefore, be regarded as a 
special American development of the African Conyzinse type 
either from Conyza or more probably by parallel evolution from the 
Erigeron type of the Heterochrominae. 
The data available for the determination of the date of origin 
of the Astereae include the occurrence of Eupatorium in the middle 
Pliocene of Europe. The origin of the Eupatorieae from the 
Heterochrominae may be considered proved, so that the origin of 
the Homochrominae is thrown back a considerable time to allow 
for the wide dispersal and differentiation of the various types. The 
fossil record of authentic Astereae is very meagre and does not 
extend beyond the Glacial Epoch. Several of theachenes from the 
lower Oligocene and upper Miocene are of the Asterean type. 
Taking into account the wide dispersal of not a few genera of the 
tribe and the wide dispersal also of the derivative Eupatorieae, 
as well as the considerable differentiation of types in the Astereae 
and Eupatorieae, the date of origin for the Homochrominae can 
scarcely be later than the middle Eocene, and might well be 
earlier still. 
Eupatoriece. The hypothesis of the origin of the Eupatorieae 
from the Heterochrominae has been more or less confirmed by all 
the characters of the group. The basal sub-tribe is the Ageratinae 
and the basal group of this sub-tribe is the Eupatorium-Mikania 
plexus. The affinity of Eupatorium with the Heterochrominae in 
styles, stamens, corolla colour, pappus, involucre, receptacle, and 
to a lesser degree the phyllotaxis, is distinct. The difference in the 
styles is the difference between type II and type Villa, and the 
chief distinction is in the complete absence of ray florets. Turning 
again to the above-mentioned key to the British flora we find that 
the discoid Aster Linosyris is distinguished from Eupatorium by the 
colour of the corolla and the shape of the leaves, characters which 
are in most cases of only specific rank. 
The reduction in the complexity of the floral structures extends 
to the stamens in the Piqueriinae (see Chap. II, E). The basal 
genus of this sub-tribe is Adeuostemma, with its one wide spread 
species A. viscosum. The basal genus of the Adenostylinae is 
probably the American genus Brickellia ; Adenostyles, however, 
shows an approach to the Cacalia type of Senecio in habit and 
involucre (cp. Bentham, I, 7, p. 401). As Adenostyles is chiefly 
developed in the Eur-Asiatic region this case requires further 
investigation ; the genus may .be a development of the Ligularia 
