8 INTRODUCTORY [pt. i 



also that in view of the time available there is no need for rapid 

 dispersal. The various causes are then considered that may help, 

 or far more often hinder, dispersal, e.g. purely physical barriers 

 like seas or mountains, barriers partly physical, partly dependent 

 upon the constitution of the plant, like changes of soil or of 

 climatic factors, or barriers (or aids to spread) dependent wholly 

 upon the latter, like the fact that herbs may spread much more 

 rapidly than trees, that parasites can only spread with their 

 hosts, that a plant may or may not spread quickly according to 

 the particular society of plants with which it meets, and so on. 

 The general impression is that dispersal in nature, except in a 

 few (probably very few) cases, must be an exceedingly slow pro- 

 cess. Only in cases where man has interfered is there much 

 evidence of rapid spread, and the popular impression that this 

 is general cannot be justified by any of the facts at our disposal 

 as to plants in unchanged natural conditions. 



Passing on to the consideration of Age and Area itself, in 

 Chapter vi, it is pointed out that when I began to investigate 

 the flora of Ceylon, I soon noticed the extraordinary differences 

 in area occupied that were to be found in species of the same 

 genus, where there were no characters of difference that could, 

 by any stretch of imagination, be regarded as fitting or unfitting 

 them for the struggle for existence. Endemic or purely local 

 species very rarely occupied the whole island, and must evi- 

 dently be adapted, if adapted at all, to local conditions within 

 its area. This led to a careful study of areas, and it was found, 

 for Ceylon, New Zealand, and elsewhere, that those species were 

 the most widely distributed in a country which had the widest 

 distribution outside, while the local or endemic species showed 

 the smallest areas of distribution; in both cases working always 

 with averages of ten allied species. 



Dividing the species of a country into classes according to the 

 amount of area occupied, it was found that the endemics were 

 most numerous in the lowest class (smallest areas), the numbers 

 decreasing steadily upwards, while the widely distributed species 

 were arranged in the exact reverse direction. Such facts were 

 much opposed to the supposition that endemics Avere adapta- 

 tions to local conditions, and equally so to the other supposition 

 that they were relics. The facts call for a mechanical explanation, 

 and the most reasonable seems to be that area occupied on the 

 average increases with age, independently of the origin of the 

 species. Endemic species are usually young beginners. 



