52 GLIMPSES OF INDIAN BIRDS 



hawk in appearance, so much so that it affords the 

 stock example of aggressive mimicry among birds. 

 Says the Wallaceian : " This cuckoo resembles a 

 hawk so closely that small birds mistake it for one. 

 When the nesting babblers see it, they flee for their 

 lives, and the cuckoo — ^the ass in the lion's skin — 

 seizes the opportunity to deposit an egg in the mo- 

 mentarily deserted nest. The strange egg is not noticed 

 by the babblers on their return because it is blue like 

 theirs. We thus see how natural selection has brought 

 about the hawk-like appearance of the brain-fever 

 bird, and caused the egg to become blue." If all 

 cuckoos parasitic on babblers were like hawks in 

 appearance, I should have nothing to urge against 

 the above explanation. Unfortunately for the Walla- 

 ceians, the pied crested and other cuckoos, which do 

 not look in the least like hawks, successfully dupe the 

 seven sisters. It would seem, therefore, that this 

 elaborate disguise of the hawk-cuckoo is quite un- 

 necessary. I grant that it may make very smooth the 

 path of the brain-fever bird. This, however, is not 

 enough. As I have repeatedly said, almost I fear ad 

 nauseam, natural selection cannot be said to have 

 brought about a structural peculiarity which is proved 

 to be merely useful, and not essential. Unless it can 

 be shown that, but for a certain peculiarity, a species 

 would have perished, it is incorrect to speak of natural 

 selection as having fixed that characteristic in the 

 species by eliminating all individuals that did not 

 possess it. Moreover, if it can be shown that any 

 specified, character has such a survival value, the 



