90 PLANT LIFE ON THE FARM. 
But apart from this external conflict with the elements, 
plants are always more or less ina state of internecine 
war. Plants of different kinds growing wild ina 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 eventu- 
ally 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 gregariously, like heaths 
on a moor, have the same requirements, 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 cir- 
cumstances 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. ‘T'o compass this end he grows them together, 
takes means by appropriate tillage, and by the removal 
of competing weeds, to enhance the conditions most 
favorable to their growth and to minimize the effects of 
those that are injurious. The warfare here is external 
as regards ‘‘ weeds,” it is internecine between individual 
plants of the same species and having the same require- 
