SCREAMING COW-BIRD 1Ir 
so wormed about and interlaced himself with his 
opponent that as soon as he succeeds in overcoming 
him he also must inevitably perish. Such a result 
is perhaps impossible, as there are so many causes 
operating to check the undue increase of any one 
species; consequently the struggle, unequal as it 
appears, must continue for ever. Thus, in whatever 
way we view the parasitical habit, it appears cruel, 
treacherous, and vicious in the highest degree. But 
should we attempt mentally to create a perfect para- 
sitical instinct (that is, one that would be thoroughly 
efficient with the least possible prejudice to or in- 
justice towards another species; for the preservation 
of the species on which the parasite is dependent 
is necessary to its own) by combining in imagination 
all known parasitical habits, eliminating every offen- 
sive quality or circumstance, and attributing such 
others in their place as we should think fit, our 
conception would still probably fall short in sim- 
plicity, beauty, and completeness of the actual 
instinct of M. rufoaxillaris. Instead of laying its 
eggs promiscuously in every receptacle that offers, 
it selects the nest of a single species; so that its 
selective instinct is related to the adaptive resem- 
blance in its eggs and young to those of the species 
on which it is parasitical. Such an adaptive resem- 
blance could not of course exist if it laid its eggs in 
the nests of more than one species, and it is certainly 
a circumstance eminently favourable to preservation. 
Then, there not being any such incongruity and 
unfitness as we find in nests into which other parasites 
