4i6 LIFE : OUTLINES OF GENERAL BIOLOGY 



hypotheses may serve to prompt research; and it is not too much 

 to say that there are some glimpses of light. Thus if melanins are 

 derived from amino-acids, which are constituents of proteins, we 

 may regard them as katabolic derivatives of these universally 

 present components of protoplasm. Similarly, it is scientifically 

 satisfactory when an animal lipochrome can be linked back to the 

 carotin or xanthophyll of plants. The "flavone" or "fiavonol" of 

 the wings of the Marbled-white butterfly {Melanargia galatea) is 

 probably derived by the caterpillar from the Cocksfoot or the 

 Timothy grasses on which it feeds. Some pigments may be unhesi- 

 tatingly classified as waste-products, such as the uric-acid pigments 

 (A some butterflies. The carmine that sometimes forms half of the 

 weight of a female cochineal insect {Coccus cacti) is a glucoside, 

 yielding sugar when treated with dilute acid, and may perhaps be 

 interpreted as a reserve-product. Of others, it may be said vaguely 

 that they are by-products of the everyday chemical routine (or 

 metabolism) of the body. 



USES OF COLOUR 



The common kingfisher flying swiftly upstream like an arrow 

 made of a piece of rainbow; the golden oranges amidst their 

 glossy leaves; the newly caught herring, blue and green, silvery 

 and red; the bluebells ringing by the wayside; the ripe cherries 

 gleaming like cornelians; the purple heather on the moor; the 

 poppies ablaze among the corn; the rubies and emeralds of the 

 Bird of Paradise; the golden kingcups in the ditch; the deep scarlet 

 of the Poor Man's Weather-glass; the azure of the Arion butterfly's 

 wing; the subtle iridescence of the worn shells wetted by the 

 incoming tide ; the daring display of the parrot ; the elusive browns 

 of the brooding woodcock — what does it all mean, this long gamut 

 of coloration? A joy for ever, of course; but this narrow human 

 point of view, though by no means unreasonable, is not satisfying. 

 We have seen that certain pigments, notably chlorophyll, haemo- 

 globin, and cytochrome, have a primary physiological significance 

 of the utmost importance, and that others, like the dark melanins, 

 the ruddy lipochromcs, and the green pigment of the bile, have 

 a primary physiological significance as by-products or waste- 

 products of the chemical routine (or metabolism) of the body. 

 Their presence is readily accounted for, and we can then proceed 

 to discover secondary utilities, such as protection or decoration. 

 Similarly, it is instructive to note that the fine lines that make 

 superficial interference-gratings and the delicate laminae of a 

 mollusc's shell are expressions of orderly rhythmic growth. In many 

 cases growth must proceed on a line-upon-line plan ; in many cases 



