STRICTLY INCOG. 63 



In order to understand how cases like these begin to 

 arise, we must remember that first of all they start of 

 necessity from very slight and indefinite resemblances, 

 which succeed as it were by accident in occasionally eluding 

 the vigilance of enemies. Thus, there are stick insects 

 which only look like long round cylinders, not obviously 

 stick-shaped, but rudely resembling a bit of wood in outline 

 only. These imperfectly mimetic insects may often obtain 

 a casual immunity from attack by being mistaken for a 

 twig by birds or lizards. There are others, again, in which 

 natural selection has gone a step further, so as to produce 

 upon their bodies bark-like colouring and rough patches 

 which imitate knots, wrinkles, and leaf-buds. In these 

 cases the protection given is far more marked, and the 

 chances of detection are proportionately lessened. But 

 sharp-eyed birds, with senses quickened by hunger, the 

 true mother of invention, must learn at last to pierce such 

 flimsy disguises, and suspect a stick insect in the most 

 innocent -looking and apparently rigid twigs. The final 

 step, therefore, consists in the production of that extra- 

 ordinary actor, the Xeroxylus laceratus, whose formidable 

 name means no more than ' ragged dry-stick,' and which 

 really mimics down to the minutest particular a broken 

 twig, overgrown with mosses, liverworts, and lichens. 



Take, on the other hand, the well-known case of that 

 predaceous mantis which exactly imitates the white ants, 

 and, mixing with them like one of their own horde, quietly 

 devours a stray fat termite or so, from time to time, as 

 occasion offers. Here we must suppose that the ancestral 

 mantis happened to be somewhat paler and smaller than 

 most of its fellow-tribesmen, and so at times managed un- 

 observed to mingle with the white ants, especially in the 

 shade or under a dusky sky, much to the advantage of its 

 own appetite. But the termites would soon begin to ob- 



