THE COLOURS OF ANIMALS AND PLANTS 119 



The red colour of Tubifex, for example, is asso- 

 ciated with the physiological activity of haemoglobin ; 

 the red colour here is probably disadvantageous as 

 such, but is counterbalanced by the physiological 

 utility of the pigment for respiratory purposes. A 

 similar explanation holds with chlorophyll, the green 

 colouring matter of plants. 



It is important to note that red is only red in the 

 presence of light, and that a red animal if put in a 

 dark place ceases to be red ; or if put in a green 

 light, which it is incapable of reflecting. Non-signi- 

 ficant colours "form the material out of which 

 natural or sexual selection can form significant 

 colours," and "all animal colours must have been 

 originally non-significant." 



The Direct Action of Environment. 



Distinct colour varieties occur locally among the 

 Lepidoptera, and a great prevalence of green is 

 shown by the fauna of Ceylon, not only by terrestrial 

 forms, but by echinoderms, corals, and other animals. 

 That differences in food have an effect on colour has 

 been shown by feeding the larvae of various kinds of 

 Lepidoptera on different plants : the larvae of the eyed- 

 hawk moth, the brimstone moth, and the peppered 

 moth show changes of this kind. This effect is not 

 due to the colour showing through the skin, but 

 must be effected through 1;he nervous system, the 

 particular pigment being actually built up by the 

 caterpillar. 



Pupae assume in many cases the colour of the 



