8 On Some Interactions of Orgamsms. 



adapted to its food supply and to the various drains upon 

 it that the species preyed upon should normally produce 

 an excess sufficient for the species it supports. And this 

 statement evidently applies throughout the entire scale of 

 being. Among all orders of plants and animals, the ideal 

 balance of Nature is one promotive of the highest good of 

 all the species. In this ideal state, towards which Nature 

 seems continually striving, every food-producing species 

 of plant or animal would grow and multiply at a rate suffi- 

 cient to furnish the required amount of food, and every 

 depredating species would reproduce at a rate no higher 

 than just sufficient to appropriate the food thus furnished. 



We must now point out how this common interest is nat- 

 urally subserved, — how the mutually beneficial balance 

 between animals and their food is ordinarily maintained. 



Exact adjustment is doubtless never reached anywhere, 

 even for a single year. It is usually closely approached in 

 primitive nature, but the chances are practically infinite 

 against its becoming really complete, and maladjustment 

 in some degree is therefore the general rule. All species 

 must oscillate more or less. Even the more stable features 

 of the organic environment are too unstable to allow the 

 establishment of any perfectly uniform habit of growth 

 and increase in any species. The most unvarying species 

 will at one time crowd its boundaries vigorously, and, at 

 another, sensibly recede from them. That such an oscilla- 

 tion is injurious to a species may be briefly shown. The most 

 favorable condition of a species is that in which its numbers 

 are maintained at the highest possible average limit ; and 

 this, as already demonstrated, requires that its food sup- 

 plies should likewise be maintained at the highest possible 

 limit, — that the species should, in fact, confine its appro- 

 priations to the unessential surplus of its food. But when 

 the numbers of an oscillating species are above this aver- 

 age limit, it will devour more than this surplus of its food, — 

 its food supplies will be directly lessened. On the other 

 hand, when the oscillating species falls below this limit, 

 its food supplies, reacting, of course can not increase beyond 

 the highest possible limit, but will reach it and there stop. 

 The average amount of food will therefore be less than it 



