PART I 



THE PRESENT POSITION 

 OF AGE AND AREA 



CHAPTER I 

 INTRODUCTORY 



1 HE existing distribution of a plant (or animal) upon the surface 

 of the globe, which is often a very complex phenomenon, is due 

 to the interaction of very many factors. Sometimes they are 

 inherent to the plant itself, sometimes they are incidental to its 

 surroundings, sometimes they partake of both qualities. At 

 times they may be active, at others ver}^ active, and at some 

 periods, or in some places, they may be more or less quiescent. 

 One pulls in one direction, another in another. As a plant spreads 

 from the place in which it originally commenced, therefore, it 

 comes under an ever-varying pull, causing it to spread more or 

 less rapidly, or at times not at all, according to the different and 

 ever-altering combinations of these factors — different climates, 

 different soils, different groups of plants that occupy the soil, 

 presence or absence of such barriers as are offered l)y mountains, 

 seas, changes of climate, and many other things. To all this it is 

 obvious that age must be added — the older the species is, the 

 more area Avill it have had time to cover. 



But mere spreading is not all; a species may at one time be 

 common in a certain region, and at a subsequent time may be 

 very rare, or even non-existent there. This may be due to many 

 things, for example, the arrival of a disease-organism to which 

 the plant may be very subject, and to which it falls an easy prey, 

 or which so reduces its vigour that it falls a prey to something 

 else. Or some new competitor may appear, which is so much 

 better suited to the local conditions that the first plnnt is reduced 

 to rarity or perhaps even to extinction. In this connection by 

 far the most important factors are those introduced by geological 

 and other changes. In times which, geologically speaking, were 

 but yesterday, Britain was united to the continent of Europe, 

 and the way was open for the passage of any species that grew 



