8 
On Some Interactions of Organisms. 
parasitic plants. The ideal adjustment is one in which 
the reproductive rate of each species should he so exactly 
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 suf- 
ficient 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 
naturally 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 supplies should likewise be maintained at the highest 
possible limit — that the species should, in fact, confine its 
appropriations to the unessential surplus of its food. But 
when the numbers of an oscillating species are above this 
average 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 cannot in- 
