Selection of Elementary Species 103 



always changing. No two successive years show 

 exactly the same proportions. At one time one 

 species prevails, at another time one or two or 

 more other species. The weather during the 

 spring and summer benefits some and hurts 

 others, the winter may be too cold for some, but 

 again harmless for others, the rainfall may 

 partly drown some species, while others re- 

 main uninjured. Some weeds may be seen flow- 

 ering profusely during some years, while in 

 other summers they are scarcely to be found in 

 the same meadow. The whole population is in a 

 fluctuating state, some thriving and others de- 

 teriorating. It is a continuous response to the 

 ever changing conditions of the weather. Rare- 

 ly a species is wholly annihilated, though it 

 may apparently be so for years ; but either from 

 seeds or from rootstocks, or even from neigh- 

 boring lands, it may sooner or later regain its 

 foothold in the general struggle for life. 



This phenomenon is a very curious and in- 

 teresting one. The struggle for life, which 

 plays so considerable a part in the modern 

 theories of evolution, may be seen directly at 

 work. It does not alter the species themselves, 

 as is commonly supposed, but it is always 

 changing their numerical proportion. Any 

 lasting change in the external conditions will of 

 course alter the average oscillation and the in- 



