422 LIFE : OUTLINES OF GENERAL BIOLOGY 



offence, then we must conclude that the production of electricity 

 is important as such. The sub-function has become a main function. 



The same holds in regard to the production of light by certain 

 plants and animals, which are popularly and erroneously called 

 "phosphorescent". It is very unlikely that the hght given off by 

 luminous bacteria, for instance, has any value in itself; but when 

 a deep-sea fish shows a luminous organ with a lens and a dark 

 envelope which restricts the emission of the light to one direction, 

 we are naturally inclined to search for some use — though there 

 are many cases where we cannot at present safely suggest what 

 that use may be. We start, then, in our discussion of luminescence 

 with the idea that in many cases the production of light may be 

 an unimportant accompaniment of some essential metaboHsm, yet 

 that the loss of energy in this direction has been over and over 

 again independently utihsed in the everyday life of the organism 



of the animal at any rate. 



Occurrence of Luminescence. — Aristotle speaks of the lumin- 

 escence of dead fishes and damp wood, which we now know to 

 be due to bacteria and fungi respectively; and long before Aristotle 

 the fishermen must have noticed that in the summer evening the 

 oars sometimes drip sparks and the breaking waves may gleam 

 with light, which we find to be mainly due to the stimulation of 

 myriads of pin-head Infusorians called Noctilucae. In warm countries 

 the bush aflame with fire-flies must have been a familiar sight for 

 ages, and among maritime peoples the "phosphorescence" of the 

 fish hung up to dry could not escape attention. Yet the idea of 

 light as a by-product either of vital processes or of complex organic 

 substances is of recent origin. Indeed, there seem to have been 

 only two important steps before the last quarter of the nineteenth 

 century. In 1667 Robert Boyle showed that the presence of air 

 is necessary for the luminescence of damp wood and dead fishes, 

 an observation that practically proves (for us) that organic lumin- 

 escence is of the nature of an oxidation. In 1794 the not less 

 ingenious Spallanzani noticed that while dried parts of naturally 

 luminous jelly-fishes cease to give out light, they do so once more 

 If they are re-moistened. This proved that bio-luminescence is not 

 necessarily bound up with vital processes, for it often occurs in 

 organic substances which are discharged from an animal, or in 

 materials present in part of a dead organism. 



In his monograph, The Nature of Aninial Light (New York, 1920), 

 Dr. E. Newton Harvey of Princeton (whose names command 

 respect!), giv.-s a criticised list of the occurrence of luminescence 

 in no fewer than thirty-six orders of animals. We say a criticised 

 list, for it is necessary to exclude a number of misinterpreted cases. 

 Thus the luminescence on the breast feathers of individual owls 

 and some other birds is probably due to contamination by a 



