CH. V] HINDER THE DISPERSAL OF SPECIES 53 



elapsed since the Eocene period— a portion only of the time 

 during which the flowering plants have existed. It is clear that 

 delay, and not acceleration, of spread has been the rule. 



The various barriers that species may meet with are then con ■ 

 sidered, first those purely physical such as seas or mountains 

 then those due to change of climatic factors from place to place' 

 which are partly physical, partly depend upon the constitution 

 ot the plant, and lastly barriers (or at times aids to dispersal) 

 depending upon the type of vegetation into which a plant may 

 try to intrude, such as forest or open grassland, or various 

 associations of plants, some of which may suit it and some not. 

 A herb, for example, may spread ten times as rapidly as a tree. 

 The effects of specialisation in structure and function are also 

 pointed out; the more specialised a plant becomes, the more 

 limited its possible range. 



The general impression which I have tried to convey in this 

 and preceding chapters is, that until man began to interfere 

 upon the large scale with cultivation, war, and clearing, the 

 dispersal of plants from one place to another must have been a 

 matter of the most extreme slowness. 



