STKICTLY INCOG. 65 



leaf- like expansions, and looking so like the foliage on which 

 it rests that no beetle on earth can possibly detect it. The 

 more cunning you get your detectives, the more cunning do 

 the thieves become to outwit them. 



Look, again, at the curious life-history of the flies which 

 dwell as unbidden guests or social parasites in the nests 

 and hives of wild honey-bees. These burglarious flies are 

 belted and bearded hi the very selfsame pattern as the 

 bumble-bees themselves ; but their larvas live upon the 

 young grubs of the hive, and repay the unconscious 

 hospitality of the busy workers by devouring the future 

 hope of their unwilling hosts. Obviously, any fly which 

 entered a bee-hive could only escape detection and extermi- 

 nation at the hands (or stings) of its outraged inhabitants, 

 provided it so far resembled the real householders as to be 

 mistaken at a first glance by the invaded community for one 

 of its own numerous members. Thus any fly which showed 

 the slightest superficial resemblance to a bee might at first 

 be enabled to rob honey for a time with comparative 

 impunity, and to lay its eggs among the cells of the help- 

 less larvae. But when once the vile attempt was fairly 

 discovered, the burglars could only escape fatal detection 

 from generation to generation just in proportion as they 

 more and more closely approximated to the shape and 

 colour of the bees themselves. For, as Mr. Belt has well 

 pointed out, while the mimicking species would become 

 naturally more numerous from age to age, the senses of the 

 mimicked species would grow sharper and sharper by con- 

 stant practice in detecting and punishing the unwelcome 

 intruders. 



It is only hi external matters, however, that the appear- 

 ance of such mimetic species can ever be altered. Their 

 underlying points of structure and formative detail always 

 show to the very end (if only one happens to observe them) 



