54 THE STUDY OF ANIMAL LIFE CHAP. 



twelve hours of the larval state," " and this is probably 

 the true meaning of the hours during which the cater- 

 pillar rests motionless on the surface upon which it will 

 pupate." ; It appears to be certain that it is the skin 

 of the larva which is influenced by surrounding colours 

 during the sensitive period, and it is probable that the 

 effects are wrought through the medium of the nervous 

 system." 



Accepting the facts that caterpillars are subtly affected 

 by surrounding colours, so that the quiescent pupae har- 

 monise with their environment, and that the adjustment 

 has often protective value, we are led to inquire into the 

 origin of this sensitiveness. That the change of colour is 

 not a direct result of external influence is certain, but 

 of the physiological nature of the changes we know little 

 more than that it must be complex. Prof. Poulton 

 suggests that the power of producing variable colour- 

 ing arose as a constitutional variation apart from the 

 influence of the environment, that the power was fostered 

 in the course of natural selection, and that its limits 

 were in the same way more or less defined in adaptation 

 to the most frequent habitat of the larvae before and 

 during pupation. 



6. Rapid Change of Colour. For ages the chamaeleon 

 has been famous for its rapid and sometimes striking 

 changes of colour. The members of the Old World 

 genus Chamceleo quickly change from green to brown or 

 other tints, but rather in response to physical irritation 

 and varying moods than in relation to change of situation 

 and surrounding colours. So the American ' chamse- 

 leons ' (Anolis) change, for instance, from emerald to 

 bronze under the influence of excitement and various 

 kinds of light. Their sensitiveness is exquisite ; ' a 

 passing cloud may cause the bright emerald to fade." 

 Sometimes they may be thus protected, for " when on the 

 broad green leaves of the palmetto, they are with difficulty 

 perceived, so exactly is the colour of the leaf counter- 

 feited. But their dark shadow is very distinct from 

 beneath." Most of the lizards have more or less of this 

 colour-changing power, which depends on the contrac- 



