Colour, its Nature and Recognition. 27 



colour, namely, the iridescent tints, like mother of pearl or shot 

 silk, which give splendour to such butterflies, as some Morphos and 

 the Purple Emperor. These are called diffraction colours, and are 

 caused by minute lines upon the reflecting surface, or by thin 

 transparent films. These lines or films must be so minute that the 

 tiny light waves are broken up among them, and are hence reflected 

 irregularly to the eye. 



Dr. Hagen has divided the colours of insects into two classes, 

 the epidermal and hypodermal. The epidermal colours are pro- 

 duced in the external layer or epidermis which is comparatively 

 dry, and are persistent, and do not alter after death. Of this nature 

 are the metallic tints of blue, green, bronze, gold and silver, and 

 the dead blacks and browns, and some of the reds. The hypo- 

 dermal colours are formed in the moister cells underlying the 

 epidermis, and on the drying up of the specimen fade, as might 

 be expected. They show through the epidermis, which is more or 

 less transparent. These colours are often brighter and lighter in 

 hue than the epidermal ; and such are most of the blues, and greens, 

 and yellow, milk white, orange, and the numerous intermediate 

 shades. These colours are sometimes changeable by voluntary act, 

 and the varying tints of the chameleon and many fishes are of this 

 character. 



In this connection, Dr. Hagen remarks, that probably all mimetic 

 colours are hypodermal. The importance of this suggestion will 

 be seen at once, for it necessitates a certain consciousness or 

 knowledge on the part of the mimicker, which we have shown, seems 

 to be an essential factor in the theory of colouration. 



This author further says, that " the pattern is not the product 

 of an accidental circumstance, but apparently the product of a 

 certain law, or rather the consequence of certain actions or wants 

 in the interior of the animal and in its development." 



This remarkable paper, to which our attention was called after 

 our work was nearly completed, is the only record we have been able 

 to find which recognises a law of colouration. 



From what has been said of the nature of light, and the physical 

 origin of colour, we see that to produce any distinct tint such as 

 red, yellow, green, or blue, a definite physical structure must be 

 formed, capable of reflecting certain rays of the same nature and 

 absorbing others. Hence, whenever we see any distinct colour, we 

 may be sure that a very considerable development in a certain 



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