74 PLANT MIGRATION 
played its part in the building up of our vege- 
tation. 
Conditions under which rapid dispersal has 
obviously an advantage occur when by some ex- 
ceptional circumstances the natural vegetation is 
destroyed within an area, as by a flood or landslide. 
Such conditions are produced artificially each season 
over much of our own country by the operations of 
agriculture. Their results will be considered in a 
subsequent chapter. 
