52 CAUSES WHICH FAVOUR OR [pt. i 



produce upon the composition of the plant societies that occupy 

 the ground. Farrow's Avork u])on the changes in the plant 

 societies upon Cavenham heath (36) made by the exclusion of 

 rabbits may be quoted as an example. A new disease may arrive 

 in a district, and a plant that was previously very common may 

 fall an easy prey to it; the more common it is the more likely is 

 it to suffer badly. 



From a general distribution point of view, of course, geo- 

 logical changes, with the changes that they cause in climates, in 

 barriers of sea or mountain, and the like, are by far the most 

 important in this connection. They have been so fully discussed 

 in geological books that there is no need to enlarge upon them 

 in this place. 



In this connection we must briefly mention the action of man, 

 which in recent times has become by far the greatest help or 

 hindrance to dispersal, though in the consideration of Age and 

 Area we have endeavoured to deal with the vegetation as much 

 as possible as it was before his interference. By clearing of 

 forest, opening of roads, making fires, cultivating the ground, 

 introducing grazing animals, carrying seeds, voluntarily or in- 

 voluntarily, about the world, and in many other ways, man has 

 made, and is making, the most enormous differences in the 

 vegetation of the globe, sometimes faAOuring the spread of a 

 species, sometimes retarding it, sometimes destroying a species 

 in whole or in part. 



Other features, again, must be considered, which M-ould hardly 

 come under any of these heads, and yet which may make a great 

 difference in the actual spreading of species. Suppose a country, 

 comparatively empty of species, united to another by a broad 

 belt of land, which is gradually sinking. Then the first species to 

 arrive across it may reach almost the whole country at once, 

 while later ones may only reach the centre, and require to take 

 an immense period to spread about. 



Summary 

 As a rule, a new plant of a given species springs up not far 

 from its parent, so that transport is at most a few yards. Even 

 if a species only travelled a yard a year, it might in a million 

 years (a mere detail in geological time) travel from London to 

 the Shetlands, Dresden, and the Pyrenees, or on an open plain 

 might cover a million square miles. The whole surface of the 

 globe might be covered in less time than it is now supposed has 



