324 THE COLOURS OF ANIMALS 



such nerve- and muscle-fibres, together with the im- 

 pulses which pass along the one and the contractions 

 which are evoked in the other, are essentially similar 

 throughout. 



The colours of underlying structures may be made 

 use of in many cases 



It is perfectly true that the colours of underlying 

 structures may be made use of for ornamental or pro- 

 tective purposes. The red colour of our blood is 

 useless as colour in most parts of the body, but the 

 transparency of the skin has permitted it to be made 

 use of in the acquisition of * complexion ' ; and I 

 believe that I am not wrong in supposing that we are 

 still true to the preference which has doubtless en- 

 couraged the growth of this attraction. 



The same thing is true of many insects in which 

 the white colour of fat, the green colour of the blood, or 

 even of the food lying in the alimentary canal, may be 

 employed in the production of a protective appearance 

 (see p. 39, also pp. 79, 80). Natural Selection has ren- 

 dered these ready-made colours available by making 

 the superficial parts transparent, and in many cases 

 such stint have been deepened or outlines strengthened 

 by the appearance of true pigment in the skin. But 

 these admitted facts do not support the theory that there 

 is any necessary relationship between superficial pig- 

 ment and the organs or structures which lie beneath. 



