On Some Interactions of Organisms. 11 
cause this would require greater activity — a higher in- 
dividuation — and this is contrary to the law that individ- 
uation and genesis are antagonistic. Instead of being 
more active than B, it will then be less active, and will, 
therefore, deposit more eggs in each caterpillar. B , how- 
ever, cannot have acquired the habit of depositing too few 
eggs in each caterpillar, as that would compel it to search 
habitually for a greater number of larvae than necessary 
— to have acquired, that is, a habit of wasting energy — 
which is, as already said, contrary to evolution. A will 
therefore, sometimes deposit too many eggs in a caterpil- 
lar, and will then either lose the whole deposit, or bring 
forth a weakened offspring, which will, in the long run, 
give way to the more vigorous progeny of B . This regu- 
lar production of a wasted excess will constitute an un- 
compensated drain upon variety A, which will end, like 
any other radical defect, in its yielding to its better-ad- 
justed rival. 
Or if, notwithstanding the foregoing, we suppose this 
excessive reproductive rate to have become fully estab- 
lished, then the parasite-ridden species will evidently 
labor under such a disadvantage in the struggle for exist- 
ence that it will probably be crowded out, in time, by some 
more fortunate rival. If the pair are permanently ill-ad- 
justed, so that permanent loss of numbers follows, they 
will be treated by the laws of natural selection as a single 
imperfect animal — they will be pushed to the wall by 
some better-adjusted caterpillar and parasite, or by some 
insect free from troublesome companions. We may be 
sure, therefore, that, as a general rule, in the course of 
evolution, only those species have been able to survive 
whose parasites, if any, were not prolific enough sensibly 
to limit the numbers of their hosts for any length of time. 
We notice incidentally that it is thus made unlikely 
that an injurious species can be exterminated, can even 
be permanently lessened in numbers, by a parasite strict- 
ly dependent upon it — a conclusion which remarkably 
diminishes the economic role of parasitism. The same 
line of argument will, of course, apply, with slight modi- 
fications, to any animal or even to any plant dependent 
