IN DEFENSIVE COLORATION 305 



sedentary organisms, a response nevertheless necessary 

 when the individual may find itself placed in any one 

 out of several differently coloured environments. In 

 such cases the adjustment is comparatively slow and the 

 result long-continued, so that the individual can only 

 adapt its colours once, or at most a few times in the 

 course of its life. 



Rapid Adjicstable Protective Resemblance is suited to 

 wandering forms which must in the course of their lives 

 continually pass and repass over environments of different 

 colours. It is widely found in fish, and also in Amphibia 

 and Reptilia, the chameleon affording the classical 

 example. It is also well known to exist in Crustacea 

 and Cephalopoda (Cuttlefish). All these rapid changes 

 of colour are due to modifications in shape or position 

 of superficial pigment cells controlled by the nervous 

 system. That this control is itself regulated by the 

 stimulus of reflected light, through the medium of the 

 eye and optic nerve, has been proved in many cases. 



Slow Adjustable Protective Resemblance is found in 

 sedentary animals passing their life in a single environ- 

 ment which, however, may be very different in the case 

 of different individuals. Thus, in many a species of moth 

 the eggs are deposited now on trees with twigs of one 

 colour now of another, on young trees or on those with 

 old lichen-encrusted branches. One caterpillar will wander 

 on to a surface of one colour, another on to a surface of 

 a very different colour, in order to become a chrysalis. 

 In all such cases the power of colour-adjustment is only 

 needed once, or at most, in certain wandering larvae, two 

 or three times. 



Such changes, so far as they have been studied, appear 

 to be produced through the nervous system, although 

 the stimulus of light probably acts on the skin, and not 

 through the eyes. Parti-coloured surfaces do not pro- 

 duce parti-coloured chrysalises, probably because the 

 antagonistic stimuli neutralize each other in the central 

 nervous system, which then disposes the superficial 

 pigments so that a neutral or intermediate effect is 

 produced over the whole surface. 



POULTON X 



