MIMICRY. 235 



that the external conditions are so rigid as to limit 

 the visual organs to one or two lines, it would 

 seem to be a warrantable assumption that there is 

 some internal condition which regulates the struc- 

 ture of such organs. 



Mimicry. 



Numerous other instances of independently ac- 

 quired organs could be given, but none so striking 

 as the one mentioned above, while many of them 

 are readily understood upon purely physical laws. 

 The likeness in shape between the wings of the bird 

 and the butterfly, for example, is the result of the 

 advantage which this shape possesses for an organ 

 of flight. 



But an interesting series of independently ac- 

 quired likenesses must be mentioned, not because it 

 presents any great difficulty, but rather because it is 

 the best illustration of the action of natural selec- 

 tion. Reference is made to the subject of mimicry. 

 Many animals, particularly insects, possess the form 

 and appearance of other animals for the protection 

 thus afforded. Some flies resemble wasps, and 

 doubtless many thus escape destruction by being 

 mistaken for the more dangerous insects. Certain 

 butterflies seem to be distasteful to insectivorous 

 birds, and others, which are not distasteful, acquire 

 the same form as distasteful species, and thus the 

 birds pass them by. Beetles imitate ants for similar 

 purposes, and many animals have a similar pro- 

 tective resemblance to inanimate objects. Our 

 grass snakes are green ; one butterfly, with its wings 



