22 THE DISPERSAL OF PLANTS [pt. i 



the world, but they have also tlie largest number in most large 

 sections of it, e.g. the Tropics, or the islands of the world, taken 

 together. This fact goes to show that dispersal has not altogether 

 depended upon the possession of a good "adaptation" for 

 the purpose, and also that when one takes large numbers and 

 long periods, it is to a marked degree mechanical. Attention was 

 first called to this striking fact by Hooker in 1888 (56, p. Ixiv), 

 in these words "the conditions which have resulted in Mono- 

 cotyledons retaining their numerical position of 1 to 4 or there- 

 al^outs of Dicotyledons, in the globe, and in all large areas 

 thereof, are, in the present state of science, inscrutable." 



If the methods of dispersal be compared throughout a family, 

 it Avill be found that they are often attached only to a genus or 

 group of genera, and thus are probably comparatively modern. 

 Even in Compositae, Avhich as a whole have the same mechanism, 

 there are a good many widely dispersed forms with no pappus. 

 "Of the Compositae common to Lord Auckland's group, Fuegia, 

 and Kerguelen's Land, none have any pappus at all ! Of the 

 many species xvith pappus, none are common to tw^o of these 

 islands" (55a, p. xxi, note). " Phyllanthiis shows by its distribu- 

 tion in the Pacific that dry-fruited Euphorbiaceae are as widely 

 distributed and as much at home as the fleshy-fruited ones" 

 (44, p. 325). And of. 7, p. 573. 



Summary 

 It being generally agreed that plants dispersed over large 

 areas began upon smaller, a study of the methods of dispersal 

 must form an introduction to that of distribution in general, 

 and a number of cases of such inA'cstigation, ffom the flora found 

 in the pollard-willow trees near Cambridge to the new flora of 

 the island of Krakatau, are given. The general results that seem 

 to come out of all such work are (1) that barriers to spreading 

 produce very important results; (2) that most individual plants 

 travel (to anything more than the very smallest distance) by 

 aid of the "regular" mechanisms for dispersal by wind, water, 

 or animals (or vegetative reproduction); but (3) that a great 

 many species are sometimes, even if very rarely, carried by 

 various "irregular" methods — mud on birds' feet, hurricanes, 

 floating logs, etc.; (4) that the distance covered is usually very 

 small; but (5) that dealing with large numbers and long periods, 

 the general result tends to be much the same in all cases under 

 somewhat similar conditions. On the other hand, the fixity of 



