CONCERNING THE CUCKOO 119 



incident in a peculiar course of development rather 

 than the cause of the development. The habit 

 probably had its beginning in either one of two 

 tendencies common among birds viz., the inclination 

 to steal from each other nest-building materials and 

 the disposition to lay occasionally in each other's 

 nests. Of the habit of stealing nest-building material 

 we have a familiar example in the rooks, and it is 

 widely distributed among birds. The Baltimore 

 oriole, a near relative of the American cow-bird, 

 is described as being very active in appropriating 

 materials collected by other birds. Some of the 

 cow-birds either build a nest of their own or seize 

 pne belonging to another bird, while in others, as 

 already mentioned, the cuckoo's habit is developed. 

 On the other hand, we have many familiar examples 

 of the habit of occasionally laying in other nests, 

 especially among the gallinaceous birds. 



It is not difficult in either case to imagine how 

 the present habit of the cuckoo was developed, 

 doubtless by easy stages. If the cuckoo of the 

 present day finds great difficulty in feeding itself 

 during its stay with us, it was obviously a great ad- 

 vantage for the voracious young bird to be entrusted 

 to the care of foster-parents. The young birds 

 developing from eggs which chanced to be deposited 

 in the nests of other birds, stood a much better 

 chance of survival, and this chance was further 

 increased when but a single egg was laid in each 

 nest. From a small beginning the habit would, 

 in fact, be developed and perfected by the operation 

 of natural selection alone. 



There remains to be mentioned what is perhaps 

 the most interesting example of gradual adaptation 



