112 PLANT-LIFE ON LAND [ch. 



movement Putting on one side the spontaneous 

 impulse of mechanical ejection as effective only 

 within limited range, the most important agencies of 

 transport are by wind, by water, and by animals. As 

 a rule the consequences of their action are not very 

 obvious owing to the ground being already tenanted 

 by a mixed vegetation. It is only where a perfectly 

 sterile area has to be peopled with a new covering of 

 vegetation, or when a new organism gains access to 

 an area previously untenanted by it, that the results 

 can be fully appreciated. An experiment upon the 

 grand scale was made in the formation of a completely 

 new Flora of the Island of Krakatau, and fortunately 

 its results were followed by competent observers who 

 kept careful records. These form the best authentic 

 story of the natural formation of a plant-population 

 where none existed before. 



Up to 1883 the islands forming the small group in 

 the Sunda Strait between Java and Sumatra, of Avhich 

 Krakatau is the largest, were covered by dense 

 vegetation. From May to August of that year suc- 

 cessive volcanic eruptions resulted in the complete 

 stei-ilisation of the surface, which was covered by hot 

 stones and ashes. Thus on cooling an miinhabited 

 desert was exposed, lying at a distance of fully 12 

 miles from the nearest living vegetation. In the 

 intervening 25 years a new Flora has sprung up upon 

 the islands. This has been studied at intervals : thQ 



