10 On Some Interactions of Organisms. 



ties, as compared with other species more exactly adjusted, 

 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 suffer, and in 

 the long course of evolution will almost certainly disap- 

 pear. The fact of survival is therefore usually sufficient 

 evidence of a fairly complete adjustment of the rate of 

 reproduction 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 hymenopterous 

 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 gradually dimin- 

 ished. 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 development. 

 The multiplication of the parasites will thus be checked, 

 and their numbers will finally become so far reduced that 

 less than the then excess of caterpillars will be infested 

 by them, in which case the caterpillars will commence 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 (^) is dis- 

 tinguished by a tendency to an excessive reproductive rate, 

 while the other (_5) mviltiplies no faster than is consistent 

 with the best interest of its host. ^, producing more eggs 

 than B^ must either parasitize more caterpillars than B-, 

 or must deposit a greater number of eggs in each. It can 

 not parasitize more caterpillars than B^ because this 

 would require greater activity, — a higher individuation, — 



