On Some Interactions of Organisms. 11 



and this is contrary to the law that individuation and gen- 

 esis 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^ however, can not have 

 acquired the habit of depositing too few eggs in each cater- 

 pillar, 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 caterpillar, and will then 

 either lose the whole deposit, or bring forth a weakened 

 oft'spring, which will, in the long run, give way to the more 

 vigorous progeny of B. This regular production of a 

 wasted excess will constitute an uncompensated drain up- 

 on variety A^ which will end, like any other radical de- 

 fect, in its yielding to its better-adjusted 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 la- 

 bor 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 strictly 

 dependent upon it, — a conclusion which remarkably dimin- 

 ishes the economic role of parasitism. The same line of 

 argument will, of course, apply, with slight modifications, 

 to any animal or even to any plant dependent upon any 

 other animal or any other plant for existence. 



From the foregoing argument we conclude that, since 



