170 TROPICAL NATURE, AND OTHER ESSAYS. 



changes are quoted by Mr. K. Meldola, in a paper on 

 Variable Protective Colouring in Insects {Proceedings 

 of the Zoological Society of London, 1873, p. 153), and 

 some of them may perhaps be due to a photographic 

 action of the reflected light. In other cases, however, it 

 has been shown that green chlorophyll remains unchanged 

 in the tissues of leaf-eating insects, and being discernible 

 through the transparent integument, produces the same 

 colour as that of the food plant. 



In the case of all these insects, as well as in the great 

 majority of cases in which a change of colour occurs in 

 animals, the action is quite involuntary ; but among 

 some of the higher animals the colour of the integument 

 can be modified at the will of the individual, or at all 

 events by a reflex action dependent on sensation. The 

 most remarkable case of this kind occurs with the 

 chameleon, which has the power of changing its colour 

 from dull white to a variety of tints. This singular 

 power has been traced to two layers of movable pig- 

 ment-cells deeply seated in the skin, but capable of being 

 brought near to the surface. The pigment-layers are 

 bluish and yellowish, and by the pressure of suitable 

 muscles these can be forced upwards either together or 

 separately. When no pressure is exerted the colour is 

 dirty white, which changes to various tints of bluish, 

 green, yellow, or brown, as more or less of either pig- 

 ment is forced up and rendered visible. The animal is 

 excessively sluggish and defenceless, and its power of 

 changing its colour so as to harmonise with surround- 

 ing objects is essential to its safety. Here too, as 

 with the pupa of Papilio Nireus, colours, such as 

 scarlet or blue, which do not occur in the immediate 



