XXI THE AGE OF COMPOSITE 239 



hill-tops, often forms the greater part of the forest above 500 feet, 

 and reaches the highest peaks of the island (2,250 feet). 



In discussing the probable mode of dispersal of these early 

 Composite plants of the Pacific we shall be treading on somewhat 

 debatable ground. We will, however, point out that the mere 

 possession of structures that could be utilised for dispersal of the 

 seeds is not the only important question here involved. If we could 

 demonstrate that all these genera possess exceptional capacities for 

 distribution over the ocean, we should prove too much, since the 

 process has been in the main suspended for ages. If, on the other 

 side, it could be shown that their fruits are not at all suited for 

 such dispersal, we should prove too little, since the ancestors of 

 these genera must have been transported to these islands in some 

 fashion or other. This clearly indicates that other important 

 factors have also come into play in determining the distribution of 

 the early Compositae of the Pacific islands. 



It was long ago pointed out by De Candolle that the possession 

 of a pappus does not, as a rule, increase the area of a Composite 

 plant, although as regards hooks and barbed appendages, such as 

 occur in Bidens, the greater areas of the plants thus provided may 

 be, as he thought, in some measure explained. Even in respect to 

 hooks and barbs it would be easy to point to cases where, as 

 Bentham remarks, unusual powers of adherence are by no means 

 indicative of wide dispersal in all cases. In any event it will be 

 also incumbent on us to explain why these genera no longer possess 

 facilities for distribution. This suspension of the means of dis- 

 persal is not, however, peculiar to the age of the endemic genera of 

 the Pacific islands. It is a character but in a less degree of the 

 succeeding age, the age of genera found outside the group, but 

 represented within it by endemic species ; and from this we may 

 suspect that we have had in operation in the Pacific an influence, 

 far-reaching both in time and space, to which the agencies of 

 dispersal have been compelled to adapt themselves, an influence 

 which has acted as a distributor of the distributing agencies. 



Coming to the fitness for dispersal of the achenes of the early 

 Composite genera of the Pacific islands, it will be assumed that 

 they have been, as a general rule, transported in birds' plumage. 

 The fruits are usually 2'5 to 12 millimetres (yV to | inch) in length, 

 and are provided either with a pappus of soft or stiff bristles, or 

 with awns or teeth, but these appendages vary much in size in the 

 different genera and in different species of the same genus. The 

 instance of Lipochzeta is especially significant as indicating the 



