302 THE WORLD OF LIFE 



is the fact that the wonderful metallic colours of so many- 

 butterflies are not caused by pigments, but are " interference 

 colours " produced by fine striae on the surface of the 

 scales. Of course, where eye-spots, fine lines, or delicate 

 shadings adorn the wings, each scale must have its own 

 special colour, something like each small block in a mosaic 

 picture. 



As this almost overwhelming series of changing events 

 passes before the imagination, we see, as it were, the gradual 

 but perfectly orderly construction of a living machine, which 

 at first appears to exist for the sole purpose of devouring 

 leaves and building up its own wonderful and often beautiful 

 body, thereby changing a lower into a higher form of proto- 

 plasm. Its limbs, its motions, its senses, its internal 

 structure, are all adapted to this one end. When fully 

 grown it ceases to feed, prepares itself for the great change 

 by various modes of concealment — in a cocoon, in the 

 earth, by suspension against objects of similar colours, or 

 which it becomes coloured to imitate — rests awhile, casts its 

 final skin, and becomes a pupa. Then follows the great 

 transformation scene, as in the blow-fly. All the internal 

 organs which have so far enabled it to live and grow — in 

 fact, the whole body it has built up, with the exception of a 

 few microscopic groups of cells — become rapidly decomposed 

 into its physiological elements, a structureless, creamy but 

 still living protoplasm ; and when this is completed, usually 

 in a few days, there begins at once the building up of a 

 new, a perfectly different, and a much more highly organised 

 creature both externally and internally — a creature compar- 

 able in organisation with the bird itself, for which, as we 

 have seen, it appears to exist. And, in the case of the 

 Lepidoptera, the wings, far simpler in construction than those 

 of the bird, but apparently quite as well adapted to its 

 needs, develop a more or less complete covering of minute 

 scales, whose chief or only function appears to be to paint 

 them with all the colours and all the glittering reflections of 

 the animal, the vegetable, and the mineral kingdoms, to an 

 equal if not a greater extent than in the case of the birds 

 themselves. The butterflies, or diurnal Lepidoptera alone, 

 not only present us with a range of colour and pattern and 



