154 THE COLOUKS OF 



The difference between slow and rapid adjustment 

 of colour 



The essential difference between the two kinds of 

 adjustment is that, in the one case, the pigmented 

 part of certain cells contracts in obedience to nervous 

 stimuli, and thus alters the general appearance ; while 

 in the other case the coloured part is actually built 

 up of the appropriate tint, or loses its colour alto- 

 gether and becomes transparent in obedience to the 

 same stimuli. The frog or fish has a series of ready- 

 made screens which can be shifted to suit the environ- 

 ment ; the insect has the power of building up an 

 appropriate screen. In many cases, however, the 

 green colour of caterpillars is due to the ready-made 

 colour of the blood, which becomes effective when 

 pigment is removed from the superficial cells, but 

 which disappears when the latter are rendered 

 opaque. Here, however, the superficial cells form the 

 screen which has to be built up or from which the 

 colour must be dismissed ; and in certain species 

 even the colour of the blood is entirely changed in 

 the passage from a green to a dark variety or vice 

 versa. 



Hence it is to be expected that the changes occur- 

 ring in an insect will occupy a considerable time as 

 compared with those which take place in a frog. 

 Another difference between the two processes is that 



