PHYSIOLOGICAL 431 



same. This seems a good working h5^othesis for this particular 

 case. 



Summary. — Bio-luminescence in animals is sometimes extrinsic, 

 that is, due to the intense metabolism of luminous bacteria on the 

 surface or even in the body. In some cuttlefishes, for instance, the 

 evidence points to the conclusion that the luminous organ is a 

 nest of symbiotic luminous bacteria; but the demonstration of this 

 in cultures has not succeeded. 



The evidence is strong that in the great majority of cases the 

 bio-luminescence of animals is intrinsic, i.e. a part of the meta- 

 bolism, sometimes slight, sometimes emphasised. In crustaceans 

 like Cypridina, in Pholas, and in almost all luminous fishes the 

 bio-luminescence seems unassociated with bacteria. 



In fire-flies, the boring Pholas, and the small marine crustacean 

 C5rpridina, the experimental evidence is strong in support of the 

 view that a somewhat protein-like ferment, luciferase, varying in 

 different types, brings about the rapid oxidation of a somewhat 

 peptone-like photogenic substance, luciferin, which changes into 

 oxy-luciferin and in so doing produces light. 



In one set of cases, e.g. sea-pens, Pholas, prawns, and the Malaco- 

 cephalus above described, the photogenic material is oxidised 

 outside the body (extra-luminescence). In the other cases, e.g. 

 Noctiluca, small marine crustaceans, the photogenic material is 

 oxidised in situ (intra-luminescence). 



The uses of bio-luminescence are still very uncertain, but it 

 looks as if a by-product, one might almost say by-path, in meta- 

 bolism had been seized upon in the course of evolution and utilised in 

 various ways — as a sex-signal, as a light to guide, as a lure for booty, 

 or as a puzzling concealment or even perhaps as an alarming warning. 



THE COLOUR OF THE HAIR.— As a special case of colour 

 interest let us take the hair. There is a considerable range of 

 colouring in the hair of mammals and of man, but it seems to 

 depend almost wholly on there being less or more of a dark pigment 

 called melanin. We include, of course, under "less or more" any 

 inequalities in the distribution of the pigment in different parts 

 of the body, or in different parts of the same hair, or at different 

 times in the year or in the life. 



A black cat has much melanin, a polar bear has little. Reynard 

 the fox is midway between these extremes. But the colour of the 

 hair almost always depends on the amount of melanin, which is 

 also the skin pigment that makes the colour-difference between 

 white, yellow, and black races, or between the blonde and the 

 brunette. Melanin is a complex substance containing carbon, 

 hydrogen, oxygen, nitrogen, sulphur, and a slight trace of iron. 

 It occurs along with a very resistant xmpigmented protein, and its 



