138 SURVIVAL OF THE FIT 



watch the changes in vegetation that occur in the cultivated field 

 that is allowed to remain fallow for several years. Each suc- 

 ceeding year will mark the crowding out of certain forms by 

 plants that possess variations that better adapt them to the lo- 

 cality. These changes in the vegetation will go on until the 

 forms are introduced whose special variations adapt them to the 

 conditions of the field and to the competition of the other plants. 

 The vegetation of the field is now fixed owing to the survival of the 

 fit. In this way the distribution and association of the vegeta- 

 tion of the earth has been brought about. 



