PT. I, CH. vj THE DISPERSAL OF SPECIES 33 



One must again multiply by say 150, to allow a seed for every 

 three mches square, or probably by even more than this. Unless 

 therefore, a seed just happens to fall on the exact spot where it 

 can grow the chance that the plant .will ever travel more than 

 a few yards from its parent is but a small one^; and the majority 

 ot plants cannot m any case travel more than a few yards 

 except by irregular aid, for want of a suitable mechanism 



Of course, m cases where a wall of uniform vegetation, like 

 the edge of a pme forest, is advancing, the number of seed re- 

 quired per tree to reach to a considerable distance will be much 

 reduced, or even where the plant, as is more often the case is 

 thinly scattered along a given front, but in any case, to reach 

 a favourable spot at some distance away, a vast number of seed 

 will be required. In the temperate zone, where seed may survive 

 for a long time, the chance of such success is greater, but in the 

 tropics, where they rarely remain ^•iable for long, is but slight. 

 Not only so, but the vegetation of the wetter tropics is usually 

 forest, and so thick that a seed dropped near the top of the tree 

 canopy will be unlikely to reach the soil if not very heavy, 

 unless by mere chance. 



There is not the least need, when one has regard to the vast 

 periods of time that are available for the purpose, for rapid 

 dispersal. Few people, perhaps, have fullv grasped the fact that 

 while some species occupy very large areas, the bulk of them do 

 not, and the average area is but comparativelv small. Upon 

 50 milhon square miles of land there are about "100,000 species 

 of flowering plants, so that if each occupied its own area, and 

 alone, the average would be about 300 square miles. But in 

 fact, at a rough and fairly hberal estimate, there are say 3000- 

 4000 in any given country, which would make the average about 

 a million square miles, probably an overestimate. But taking it 

 at a million for convenience, this area could be covered in a 

 million years (a mere detail in geological time) by an annual 

 plant Avhich merely moved forward a yard a year, and which 

 started on an open plain of the necessary size, with a uniform 

 chmate. 



While the radius of the area occupied increased 1, 2, 3, 4, 5, 

 etc., the area {nr^) would increase 3, 12, 27, 48, 75, 108, the 

 differences being 9, 15, 21, 27, 33, or an annual increase of G. 



1 The rapid spread of weeds does not affect this argument, for tJiey are 

 spreading upon cultivated ground, and owe their rapid dispersal to changed 

 or unnatural conditions, as do the introductions considered in Chapter in. 



W. A. o 



