94 PLANT LIFE ON THE FARM. 



plants are always more or less in a state of internecine 

 war. Plants of different kinds growing wild in a state 

 of nature may contend one with another for root-hold, 

 soil-food, and for space to expose their foliage to the sun. 

 Under such circumstances, if there is enough for all, it may 

 be that the severity of the struggle may be slight, owing to the 

 different requirements of the different plants, but even then 

 the stronger of the two will eventually prevail. A farmer, 

 however, would hardly call the preponderance of weeds an 

 instance of the survival of the fittest. From his point of 

 view it would certainly not be so, however true it might be 

 in wild nature. Plants of the same kind growing gre- 

 gariously, like heaths on a moor, have the same require- 

 ments, and these are supplied in about equal proportions 

 to all the individual plants. The result is that while the 

 weak ones are crowded out, the survivors are all pretty 

 much on an equality ; but once the balance is destroyed, 

 then that which is the stronger, or the one best adapted 

 to the circumstances under which it is placed, will survive. 

 In cultivation we have illustrations of mixed and of 

 gregarious vegetation in the sense above employed, as 

 well as of alternate vegetation as in the case of " rotation." 

 In the case of the cereals, of turnips, of potatoes, and others, 

 we have instances of gregarious vegetation induced, indeed, 

 by the will of the cultivator. His object is to secure 

 the most profitable development of one particular kind of 

 plant, wheat, barley, oats, or what not. To compass this 

 end he grows them together, takes means by appropriate 

 tillage, and by the removal of competing weeds, to enhance 

 the conditions most favourable to their growth and to mini- 

 mise the effects of those that are injurious. The warfare 

 here is external as regards "weeds," it is internecine between 



