28 
James Small. 
differentiation that present conditions make the Alaska-Siberian 
bridge impassable for this group. The probable use of the bridge 
before the last period of glaciation in the north is indicated by the 
distribution of Gerbera and Trixis (Pig. 35) and by the close affinity 
of the wider spread genera of both hemispheres in each of the sub¬ 
tribes which reach the Old World. 
Vemoniece. The derivation of the Vernoniinae from the 
Liabinae is confirmed by the very close proximity, if not the actual 
co-incidence, of the centres of origin of these two sub-tribes (Figs. 
30 & 36). The derivative position and relative youth of the 
Lychnophorinae are confirmed by the smaller development of this 
sub-tribe in those regions where it occurs, and by the absence of 
any considerable development to compare with the tropical 
African concentration of the Vernoniinae. 
Asterece. Various suggestions have been made for alterations 
in the positions of the sub-tribes in the Astereae (see Chapter IV, D 
and Chapter V, D). The evidence of the distribution is also some¬ 
what uncertain. The origin of the Homochrominae from the 
Senecioninae is confirmed by the co-incidence of the centre of 
origin of the former with a large concentration of the latter in 
Mexico and the U.S.A. The predominance in genera and species 
of the Homochrominae over the Heterochrominae in the region of 
origin supports the relative age of the former, while the wide dis¬ 
tribution of the latter suggests that it is the group which has given 
most of the other sub-tribes. 
The greatest development of the Baccharidinae coincides with 
a considerable development of both the basal sub-tribes (Fig. 37), 
so that the evidence of structure decides the balance in favour of 
the Heterochrominae as the source of that sub-tribe. The African 
development of the Conyzinae and Grangeinae confirms the origin 
of these sub-tribes from the Heterochrominae as suggested (Chapter 
IV, D). The Mexican concentration and wide area of the Bellidinae 
confirm the origin of this sub-tribe from the Homochrominae, as 
suggested (Chap. V, D). 
Eupatoriece. The co-incidence of the chief centre of the 
Ageratinae with one of the most important centres of the 
Heterochrominae confirms the origin of the former sub-tribe as 
suggested in Chapter IV, F). The wider area of the Ageratinae 
compared with that of the Adenostylinae or Piqueriinae and the 
co-incidence of all three centres in the Mexican region confirms the 
derivative position of those two sub-tribes. The exceptional 
