50 CAUSES WHICH FAVOUR OR [pt. i 



of Tibet, one can hardly doubt that here a very ancient type of 

 plant finds its still more ancient conditions of existence." On 

 the other hand, many species of very local range seem to be 

 suited to very local conditions, and more or less incapable of 

 further spread without further modification. Some Utricularias 

 in South Brazil, for example, are specialised to grow in Bromeliad 

 pitchers, and can only go where those exist. Copeland (18) 

 mentions the case of Stenochlaena areolaris, which is epiphytic 

 on Pandanus utilisswms only, and confined therefore to places 

 Avhere that grows. It seems not impossible that some Mesembry- 

 antheinums in South Africa are specialised to suit the exact 

 climate in which they grow, and are thus rigidly localised. 



It is thus highly probable that at times very local species may 

 in reality be much older than from the area occupied one would 

 be inclined to think. This, however, does not affect the soundness 

 of the hypothesis of Age and Area to be advanced below, but 

 merely goes to show that ecological barriers may often be very 

 effectual. 



An ecological factor which is of the greatest importance to 

 a commencing species is the type of vegetation into which 

 it is born. In the natural state of the vegetation of a country, 

 the ground in any place is covered with an assortment of plants 

 which is found to be fairly constant in its composition so long 

 as the general conditions are much the same. This assortment 

 is termed a plant society or association, and upon the chalk 

 downs, for example, or the moors of Yorkshire, one finds much 

 the same society, made up of much the same proportions of its 

 various members, in places far removed from one another. Con- 

 sequently, if a new species is evolved at a given place, and can- 

 not enter the society that exists there, it will die out again by 

 the simple action of natural selection. The instant that it is 

 produced, it will have to undergo a strenuous struggle for exist- 

 ence, but if it pass successfully through that, it may succeed, 

 and may spread with the society which it has entered, and 

 ultimately also enter other societies. 



As the number of plants, and their variety, in any society, 

 increases, the entrance of a newcomer probably becomes in- 

 creasingly difficult. The society is said to be in progress from 

 an "open" condition to a "closed" one. But as Clements has 

 said, a society is never in a state of stable equilibrium, and 

 though one may regard it as perfectly closed, it may yet be able 

 to admit new members. The most conspicuous plant upon the 



