56 BIRDS' NESTS 



preserve and extend the practice of such an action 

 if beneficial, or at least not harmful, to the species 

 concerned. We can also understand how a species 

 might derive sufficient benefit from being ousted from 

 its home by some invading form, being thus relieved 

 of the duties of incubation and rearing a brood, that 

 in time, by a similar process of selection, it might 

 entirely relinquish all inclination to perform them. 

 On the other hand, parasitism may have arisen 

 through the actions of young birds in the following 

 manner. In the first place we have the very interest- 

 ing and significant fact that some at anyrate of these 

 bird parasites are very voracious feeders. Then we 

 have the equally well-known fact that certain species 

 especially show a strong desire to feed any deserted 

 or helpless nestling that may chance to come in their 

 way — the drooping, fluttering wings, open mouth, and 

 pleading notes of such outcasts apparently exciting 

 parental instincts in the older birds, and irresistibly 

 prompting such birds to respond to them. Many 

 instances might be given of birds adopting and feed- 

 ing the deserted or orphaned and helpless young of 

 other species, and the significant result of such a 

 combination of facts is at once palpable. There 

 would also probably be a synchronous development 

 of a strong tendency in the young birds brought up 

 under such circumstances to consort with the species 

 that had befriended them, and we can then under- 

 stand the origin of the habit of seeking the nests of 



