74 THE STORY OF PLANT LIFE 



makes for such changes. Floods, silting up by 

 natural causes, rise or fall in coast level, earthquakes, 

 etc., are amongst the many natural causes. 



Man himself, however, in such relatively ancient, 

 civilised, thickly populated countries as the British 

 Isles has brought about the greatest changes. 



He has cut down the forests for tillage, fuel, for 

 ship construction, or for dwellings, etc. This has 

 reduced the area of woodland. It has also affected 

 the amount of moisture or rainfall. In place of 

 woods we now have pastures or meadows. 



By cultivation he has entirely altered the character 

 of the soil and the nature of the vegetation it sup- 

 ports. This is shown by the great prevalence of 

 ridge and furrow, dating from the Conquest or 

 earlier, when corn was cultivated more extensively 

 than to-day. The disturbance of the soil and the 

 introduction of foreign types owing to cultivation 

 has obviously brought about greater changes in the 

 natural vegetation than can be accounted for readily. 

 But, on the contrary, a good deal of this culti- 

 vated land has returned to its former state and may 

 be regarded as purely natural pasture. But the 

 majority of the natural pasture lies either above the 

 zone of cultivation, i. e. above altitudes of looo ft., 

 or occurs upon such soils as limestone or chalk 

 where the soil is shallow and has not been tilled, or 

 where, as in the last case, there was little original 

 woodland. 



A third factor introduced by man is drainage. 



