THE WAGTAILS 243 



might not be deposited during one of these, and it also seems 

 probable that, as the habit of seizing an alien nest became more and 

 more rooted, acquiring, at length, the strength of an instinct, the 

 legitimate and guilty egg would sometimes lie side by side. Indeed, 

 if I remember rightly, this is sometimes the case with the cowbird of 

 South America, or, rather, the eggs of the latter may be separated from 

 those of the rightful owner of the nest by a superimposed thin layer 

 of straw only, so that, quite conceivably, the two might sometimes be 

 hatched out together. That, in the cowbird, we have a considerable 

 approach to the cuckoo, can hardly be denied, yet the instinct of nidi- 

 ficatioii has not, in its case, been quite lost, and the same, if I mistake 

 not, is the case with some (parasitically) less developed relatives 

 of the cuckoo itself. The starling, also, as we know, often 

 appropriates the nest of the green woodpecker, and it can hardly 

 be doubted that a greater knowledge of the habits of the birds of 

 the world would supply us with other similar instances. With 

 such materials to work upon, the evolution, through natural selection, 

 of the cow-bird, and ultimately the cuckoo, does not appear to me 

 very surprising, the really surprising thing perhaps being that such 

 parasitism is so rare. But the instinct of building, which may continue 

 even after the nest is seized, with the tendency of the disturbed 

 owners to desert, help to explain this. In deference to those who 

 think it rash or wicked, or both, to cherish a theory till the facts* 

 in support of it are so overwhelmingly strong that it is hardly any 

 longer one, I would point out that they may, if they please, consider 

 this idea of mine as a suggestion only, which is something slighter 

 and as I would humbly urge in this connection less criminal than 

 a theory, or hypothesis. Should this plea be not accepted, then I 

 have only to tell them that, in my opinion, if not in theirs, the facts 

 here touched upon are quite sufficient to justify my remarks on 

 them, whatever they think these amount to. They are, in truth, 

 by no means slight, but pregnant, rather, and the cuckoo, though 

 some may be content to take her as a miracle (like the rooks and 



