20 THE DISPERSAL OF PLANTS [pt. i 



receive 137 new species in less than 40 years. As Lyell stated 

 in 1853: 



Every naturalist is familiar with the fact, that although in a 

 particular country, such as Great Britain, there may be more 

 than 3000 species of plants, 10,000 insects, and a great variety 

 in each of the other classes ; yet there will not be more than a 

 hundred, perhaps not half that number, inhabiting any given 

 locality. There may be no want of space in the supposed tract ; 

 it may be a large mountain, or an extensive moor, or a great 

 river plain, containing room enough for individuals of every 

 species in our island; yet the spot will be occupied by few to 

 the exclusion of many^ and these few are enabled, throughout 

 long periods, to maintain their ground successfully against every 

 intruder, notwithstanding the facilities which species enjoy, by 

 virtue of their power of diffusion, of invading adjacent terri- 

 tories (69, p. 670). 



This fixity of the vegetation in any given neighbourhood, 

 though familiar enough to everyday observation, tended to be 

 ignored during the period of the hunt for adaptations; but with _ 

 the rise of the study of ecology it has once more come into 

 prominence, and the tendency at present is perhaps to regard 

 it as too permanent. A given area of ground is occupied by a 

 society or association of plants, made up in a fairly definite way. 

 This association may be open, leaving room for possible new- 

 comers, but tends always to become dosed, by taking in the 

 maximum number which can mutually adjust themselves to the 

 conditions there prevailing, and as altered to some extent by 

 each new arrival. It is a matter of extraordinary difficulty for 

 a newcomer to obtain a foothold in a closed association, which 

 mav thus form an almost complete barrier to passage. But with 

 the changes brought about in the soil, etc., by the vegetation 

 itself, and for other reasons, an association sooner or later passes 

 its climax, and tends to be succeeded by others. As Clements 

 says (16), "the most stable association is never in complete 

 equilibrium"; and again, "local migration is primarily respon- 

 sible for the population of new areas... most of the evidence 

 available shows that effective invasion in quantity is always 

 local." It is clear that to think of plants in general as travelling 

 rapidly about the world by aid of their dispersal mechanisms is 

 to take a completely incorrect view of the situation. 



In fact, it is clear, and will be made clearer in the chapter upon 

 barriers, that in nature dispersal will be an extremely slow pro- 

 cess. The majority of plants have no special "mechanism" for 



