130 JOTTIXGS ABOUT BIRDS. 



Cuckoo, or what is more probable, a common 

 ancestral form, labouring under the great dis- 

 advantages of rearing from five to eight voracious 

 young, chanced to desert all or a portion of them, 

 most likely when they had left the nest, and that 

 they were adopted by other species. It is a well- 

 known fact, of which many instances might be 

 given, that various birds evince a strong desire to 

 play the part of foster-parent to deserted, helpless 

 nestlings. The Cuckoo would soon seize upon the 

 obvious advantage, and acquire the habit of regularly 

 leaving all or some of its offspring each season to 

 be thus befriended by other species. There would 

 also be a strong tendency in the offspring to 

 consort w^ith the species that had played the part 

 of foster-parent to them, and probably to lay their 

 eggs in nests belonging to that species. The habit 

 might thus have gradually developed into actual 

 and complete parasitism, the Cuckoo's parental 

 instincts gradually getting weaker and weaker, until 

 nest-buildino; and incubation, bv tlie sure vet subtle 

 working of natural selection, were finally dispensed 

 with, and the eggs invariably dropped into alien 

 nests. Of course it may be suggested, that if the 

 Petrel or the Gannet, for instance, only rear a 

 single nestling, there is nothing to prevent the 



