i6 LOST AND VANISHING BIRDS 



is caused by the greater numbers, in Paraguay, of 

 a certain fly which lays its eggs in the navels of 

 these animals when first born. The increase of 

 these flies, numerous as they are, must be habitually 

 checked by some means, probably by other parasitic 

 insects. Hence, if certain insectivorous birds were 

 to decrease in Paraguay, the parasitic insects would 

 probably increase ; and this would lessen the 

 number of the navel-frequenting flies. Then cattle 

 and horses would become feral, and this would 

 greatly alter (as, indeed, I have observed in parts 

 of South America), the vegetation; this, again, 

 would largely aflect the insects; and this, as we 

 have just seen in Staffordshire, the insectivorous 

 birds ; and so onward, in ever-increasing circles of 

 complexity. Not that under nature the relations 

 will ever be as simple as this. Battle within 

 battle must be continually recurring with varying 

 success ; and yet in the long-run the forces are so 

 nicely balanced that the face of nature remains 

 for a long time uniform, though assuredly the 

 merest trifle would give the victory to one organic 

 being over another." 



Most, if not all, organisms are therefore so 

 delicately adapted to their environment, that they 

 quickly become sensitive to the least disturbing 



