SCREAMING COW-BIRD in 



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, mfoaxillaris. 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 



