NATURAL SELECTION AS APPLIED TO BIRDS 327 



more remarkable phases of coloration. Herein quite harmless 

 and passively disposed birds have developed a superficial resem- 

 blance to species which, from one cause or another, inspire fear 

 and dread among other species ; while on the other hand, there 

 are birds which, for aggressive purposes, have assumed the 

 guise of harmless species — wolves in sheep's clothing. That is 

 to say then, we have here a number of species which, for par- 

 ticular reasons, have assumed the likeness of other birds, instead 

 of a likeness to inanimate surroundings ; they are mimics. 



The evolution of this mimicry is doubted by some, the 

 resemblance referred to being put down as accidental. Pro- 

 fessor Poulton, however, and a great many more who have 

 studied this matter, agree rather to regard the facts as the 

 result of natural selection, which has given rise on the one 

 hand to protective mimicry, and on the other to aggressive 

 mimicry. 



These two forms of coloration though "apatetic" in char- 

 acter are regarded by Professor Poulton as forming a separate 

 group — pseudosematic colours. And these he divides into 

 pseudaposematic colours — protective mimicry, and pseude- 

 pisematic colours — aggressive mimicry. 



Protective mimicry, or pseudaposematic coloration, is well 

 illustrated among birds by the Common Cuckoo (Cuculus can- 

 orus). This bird presents a really striking resemblance to a 

 Sparrow-hawk, and thereby, it is supposed, it is enabled to 

 carry on with ease its nefarious practice of putting its young 

 out to nurse. Among small birds, such as are victimised by 

 the Cuckoo, the Sparrow-hawk is greatly dreaded. So dis- 

 guised then, the male Cuckoo, when his paramour is ready to 

 dispose of an egg, hovers over spots likely to contain nests of 

 the desired foster-parents. These threatened, as they suppose, 

 by the bully of the country-side, at once commence to buffet 

 him, gaining courage, in the defence of home, which at other 

 seasons of the year they cannot command. Under this attack 

 the pretended marauder beats an affected retreat, followed by 

 his puny adversaries. When the pursuit has carried away the 

 pursuers sufficiently far the female quietly slips up to the nest 

 and then drops in her egg. On the return of the frightened 

 birds, they either fail to notice the addition to the nest, or are 

 indifferent, finding the eggs they left still whole. 



