I THE COLOURS OF ORGANISMS 15 



the sculpturing of butterflies' scales has been extolled 

 by most possessors of a microscope, while savage 

 and civilised races are alike in their admiration for 

 the feathers of birds. The fact that organisms so 

 widely separated as are birds and butterflies are 

 alike in exhibiting both exquisite structural colora- 

 tion and a wonderful development of structures 

 arising from the cuticle, suggests that the structural 

 colours are in origin merely a result of extreme 

 differentiation of the cuticle, and therefore produced 

 by the same cause which gave rise to this differentia- 

 tion. The presence of brilliant iridescence in some 

 of the mud-inhabiting worms is therefore not quite 

 inexplicable, for here also we find that the cuticle 

 shows a considerable amount of differentiation. We 

 can also further understand how it is that the highest 

 pitch of perfection is attained in birds and butterflies, 

 when we consider that in both cases the colouring' 

 occurs in connection with structures which are of 

 supreme importance to the species, that is, with the 

 feathers of the bird and the scales, which are but 

 outgrowths of the wings, in butterflies. 



Production of Light by Organisms (Phos- 

 phorescence), ITS Distribution and Meaning 



The production of light, the phenomenon commonly 

 though incorrectly spoken of as phosphorescence, is 

 so striking a characteristic of many organisms that 

 it deserves notice here in our consideration of colours. 

 It is unnecessary to enter into a detailed account of 

 the theories of Dr. Carpenter and others as to the 

 probable use of this characteristic, because these 



