326 THE WOKLD OF LIFE 



through the skin.^ Another complication 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 protoplasm. 

 Its limbs, its motions, its senses, its internal structure, are all 

 adapted to this one end. Wlien fully grown it ceases to feed, 

 prepares itself for the great change by various modes of con- 

 cealment — 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 — be- 

 come 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 comparable 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 construc- 

 tion than those of the bird, but apparently quite as well adapted 

 to its needs, develop a more or less complete covering of minute 



1 This description is from Mr. A. G. Mayer's paper on the Development 

 of the Wing Scales of Butterflies and Moths (Bull. Mus. Comp. Zool. Harv. 

 Coll., June 1896), so far as I can give it in a verv condensed abstract. 



