<^"- "] INTO NEW AREAS 19 



he studied the Dipterocarpaceae in the Botanic Gardens at 

 Singapore. These are tall trees with rather large fruit, upon 

 Avhich two or more of the persistent sepals grow out into large 

 wuigs. Falling as they do from a considerable height, and re- 

 volvmg as they fall, these fruits may be carried to some distance 

 before they drop, if there be a wind blowing. A Shorea, 100 feet 

 high, was found to scatter its fruits freely up to 40 yards dis- 

 tance, but not beyond 100. As it fruits at thirty years old a 

 httle calculation will show that in the most favourable circum- 

 stances conceivable, with the ground clear of other vegetation 

 It would take about 60,000 years to migrate 100 miles. DiiJtero- 

 carpus grandifolius, another of this family, ranges from the 

 Malay Penmsula to the Philippines, and Ridley estimates that 

 at least If million years would be needed to traverse this dis- 

 tance. He considers that liglit powder-like seed affords the most 

 rapid transit, plumed fruit or seed, hke the dandelion and other 

 Compositae, next, and winged fruit or seed, like the ash or the 

 Dipterocarps, the slowest (of the "regular" mechanisms for 

 wind-dispersal). In another paper he gives interesting points 

 about the dispersal of seed by mammals, calling especial 

 attention to the small distances usually travelled in such 

 cases. 



What has been said so far might be read to mean that dis- 

 persal of plants was always a comparati^•eIy simple and rapid 

 process, only interfered %vith to some extent"by actual barriers; 

 and it is necessary now to make clear that in nature this is far 

 from being the case. The desirability, under the Darwinian 

 theory, of finding as many, and as effectual, "adaptations" as 

 possible, has led to those for seed-dispersal receiving much greater 

 credit than is their due. In all the cases (except Ritigala) that 

 we have so far considered, the dispersal of the plants has been 

 into areas of ground that could be easily occupied, on account 

 of the lack of competition ; and the same is the case with the 

 introductions described in the next chapter. But suppose that, 

 instead of the 4000 willow-tops, one thought of 4000 areas of a 

 square yard each (or of a single acre) upon a moor or in a forest, 

 it is at once obvious, from ordinary observation, that in 100 

 years they would not receive 80 new species of plants, even 

 though these might be growing within 200 yards. It is doubtful 

 if they would even receive one or two. Nor would an area equal 

 to that of the island of Krakatau, but upon a tropical savannah. 



