THE VEGETATION OF SMALL ISLETS. 



39 



Species, " why this or that species is rare, we answer that 

 something is unfavorable in its conditions of life ; but what 

 that something is, we can hardly ever tell." And elsewhere 

 in the same classical work the author points out how " under 

 nature, the slightest difference of structure or constitution may 

 well turn the nicely balanced scale in the struggle for life, and 

 so be preserved." 



In the case of a small islet, situated at some distance 

 from the land, the struggle for existence among the plants 

 which clothe its surface must be greatly intensified, and the 

 chance of some particular species being ultimately crowded 

 out and disappearing altogether must be enormously increased. 

 The extermination of a plant, which would be only temporary 

 when the locality was part of a large area, would be permanent 

 if it occurred upon a small island. Where there is a con- 

 tinuous land area, the occurrence of some unusual climatic 

 condition, such for example as prolonged drought, might 

 produce a serious result locally ; but in all probability it 

 would not alter the character of the flora of that region as a 

 whole, because in course of time the plants would spread from 

 the adjoining country unaffected by the drought, and so 

 replenish the devastated aren. But in an islet such a drought 

 would cause the permanent extirpation of all the weaker 

 forms, and their place would be at once occupied by the 

 sturdier tenants of the soil which had survived the rainless 

 period. Those plants which were capable of withstanding 

 drought would multiply and spread, and those that could not 

 would disappear. ' Many other hostile agencies are continually 

 at work besides drought, such as the violence of wind and 

 waves, and the destruction caused by small predatory crea- 

 tures inhabiting the islet. 



In various ways, then, an isolated spot of this kind would 

 gradually be stripped of a number of plants which had 

 formed part of its natural flora when it was still united to the 

 mainland. One by one all the species that from some cause 

 or other were unable to withstand such hardships as might 

 occasionally assail the islet would slowly die out, and at last 

 only the hardiest and strongest would survive as the champions 

 of vegetation. The more limited the area of conflict, that is 

 to say, the smaller the islet, the more severe would be the 

 struggle for existence. 



It has repeatedly been demonstrated that small and 

 restricted areas, such as the little islets we are about to 

 consider, will support the largest possible amount of plant 

 life when they are occupied by very diverse forms. 



