THE WONDER OF LIFE 



541 



■VU< 



able in a short time to alter the 

 disposition of their pigment cells 

 so as to become part and parcel of 

 their background. The figures we 

 have given show the nicety of the 

 harmonious adjustment (Fig. 10). 



Great care must be exercised 

 in ascribing protective value to 

 the colour-resemblance between 

 an animal and something else, 

 and each case must be judged on 

 its own merits. It must be made 

 clear, for instance, that the re- 

 semblance which conceals the 

 creature from us is equally effec- 

 tive in conceahng it from its 

 natural enemies. The desert in- 

 sect does not escape the desert 

 lizard, and the green insect on 

 the twig is unhesitatingly picked 

 off by the sharp-eyed bird who 

 has made that its business. Some 

 creatures, hke sea-slugs, which 

 are often very harmonious with 

 their surroundings, are seldom 

 eaten by anything. 



It is satisfactory, then, that 

 there are some definite observa- 

 tions proving the protective Fig. 86. — A Venezuelan insect, 



, . , • n iTT'ii Umbronia spinosa, with 



value m particular cases. With ^ marked resemblance to 



silk threads Cesnola tethered a prickle on the twig on 



which it (um) is seated., 

 forty-five green praying mantises From ^ specimen.. 



