CH. xiii] IN THE COMPOSITAE 129 



are so marked because they are presumed to have arisen about 

 the same time. Taking these six sub-tribes as two groups we 

 get the series 6-7, 6-5, 2-7, 6-6, 6-0, 1-3. In this series there 

 is only one prominent break, 2-7 for Madiinae with only seven 

 genera. Considering the sub-tribes showing exceptional figures 

 for average generic area, there is the Coreopsidinae (9-6) with 

 17 genera of which Bidens is a very widely spread weedy type 

 with a very special dispersal mechanism, especially when the 

 early migrations of man are regarded as a means of dispersal. 

 The Melampodiinae shows a low average, but it has an average 

 generic area very similar to several of the other young sub- 

 tribes. The other exceptional figure is 12-8 for the Ambrosiinae; 

 in which there are only nine genera, of which both Ambrosia 

 and Xanthium are widely spread weeds, the latter like Bidens 

 with a special dispersal mechanism. 



Astereae. The series for this tribe is very uniform, running 5-4, 

 6-0, 7-5, 7-7, 7-0, 5-5. Such a sequence, with the most primitive 

 sub-tribe showing the lowest average generic area, might well 

 seem to show that the present thesis cannot be maintained, but 

 only two of the six groups have more than ten genera. Further, 

 the division into sub-tribes is introduced by Bentham (7, 

 p. 402) thus: 



The vast tribe of Asteroideae is neither so well marked as a 

 whole..., nor yet is it well divisible into distinct groups. Nearly 

 the whole of the 90 genera, comprising above 1400 species, pass 

 into each other through exceptional or intermediate forms.... The 

 Asteroideae not being divisible into distinct sub-tribes, we may 

 for geographical purposes consider a number of types with the 

 various divergences from them. 



Bentham also gives the key to this anomalous distribution as 

 follows: ''Aster, taken in its most extended sense, ranges over 

 the whole area of the tribe; but isolation has been ancient 

 enough to admit of its having established special forms in 

 different countries, which are now admitted as genera by most 

 botanists" (7, p. 402); and in the Solidago type (7, p. 410): 



We have here about 320 species in 24 genera, all nearly allied 

 to each other and only distinguished technically from Aster and 

 its immediate allies by the homochromous florets, the ray florets, 

 when present, being yellow, like the disk — a character in general 

 of so little value that it cannot, in Senecio for instance, be ad- 

 mitted as of more than specific importance. 



Translating these quotations from Bentham, who makes 

 several other statements of a like nature (cf. op. cit., pp. 405 



