10 On Some Interactions of Organisms. 
vantage in the search for food, and in all its other activi- 
ties, as compared with other species more exactly adjust- 
ed, or, as compared with members of its own species 
which tend to a better adjustment. As soon as a better- 
adjusted competitor appears, the other must begin to suf- 
fer, and in the long course of evolution will almost cer- 
tainly disappear. The fact of survival is therefore usual- 
ly sufficient evidence of a fairly complete adjustment of 
the rate of production to the drains upon the species. 
For the sake of illustration, let us take an instance— 
and the most difficult we can find for the application of 
these ideas — the case of a caterpillar and its hymenop- 
terous parasite. 
If the rate of increase of the parasite be relatively too 
great, that is, if more parasites are produced than can 
find places of deposit for their eggs in the bodies of the 
mere excess of caterpillars, some of them will deposit 
their eggs in caterpillars which would otherwise come to 
maturity — that is the number of caterpillars will be grad- 
ually diminished. With this diminution of their hosts the 
parasites will find it more and more difficult favorably to 
bestow all their eggs, and many of them will fail of devel- 
opment. The multiplication of the parasites will thus be 
checked, and their numbers will finally become so far re- 
duced that less than the then excess of caterpillars will be 
infested by them, in which case the caterpillars will com- 
mence to increase in numbers, and so on indefinitely. 
Briefly, the excessive rate of increase of the parasite will 
keep up an oscillation of numbers in both parasite and 
host which will cross and recross a certain average line. 
Let us now look at the method by which Nature may 
check this injurious fluctuation. 
Let us suppose two groups of a parasitic species at work 
on the same species of caterpillar, of which one (A) is 
distinguished by a tendency to an excessive reproductive 
rate, while the other (B) multiplies no faster than is con- 
sistent with the best interest of its host. A, producing 
more eggs than B , must either parasitize more caterpil- 
lars than B, or must deposit a greater number of eggs in 
each. It cannot parasite more caterpillars than B, be- 
