INTRODUCTION 73 



between plants and their attitude towards a saline 

 soil. They are either halophile or halofuge, the 

 former constituting a marked type of vegetation 

 termed halophytic. 



At one period it may be assumed that this country 

 was more or less entirely covered by virgin forest. 

 There were here and there natural open spaces, due 

 to the character of the soil, e.g. as where now it is 

 shallow, or covered by stony debris, or where the soil 

 was originally water-logged and covered by thick 

 layers of peat. But even then, as peat deposits show, 

 there were periods when forests covered such sites. 

 And this gives us a clue to the present diversity of 

 plant formations, which would appear to be inex- 

 plicable if all the country were originally woodland, 

 unless some general factors made for diversity. For 

 one type of formation arises from another. One 

 may become degenerate and give place to another, 

 as where woodland becomes scrub, or scrub pasture. 

 At the margins of different formations one type may 

 invade another, as where aquatic vegetation or con- 

 ditions invade meadow land. Thus we get a patch- 

 work arrangement of formations at any one period, 

 and by the law of migration and invasion, during 

 successive periods, variable in length, we may get a 

 succession of plant formations in any one area. 

 Such changes follow well-defined laws, and the 

 transitions from one type to another can be recog- 

 nised and explained. The variation in water content 

 due to fluctuations in the atmospheric conditions 



