532 THE POPULAR SCIENCE MONTHLY 



WHY DO CERTAIN LIVING FORMS PRODUCE LIGHT? 



By F. ALEX. McDERMOTT 



WASHINGTON, D. C. 



TO every observer of natural phenomena, the query must some time 

 come, " Why do certain living creatures produce light?" The lu- 

 minous molds of decaying wood, the photogenic bacteria of the sea-water, 

 the fire-flies and lightning bugs, the deep-sea fish and other mysterious 

 forms " that move in the waters " — why should some of them be en- 

 dowed with the property of producing light? The question is undoubt- 

 edly one of fundamental biologic importance. The production of light 

 by living forms is really no more wonderful than the production of heat, 

 motion or electricity, but the production of heat and of motion is so 

 common and so well known that but little attention is paid to them, 

 while the forms which produce electricity are relatively so scarce that 

 they are little known outside of the scientific world. Between these 

 classes are the forms possessing the photogenic function — sufficiently 

 common to be well known almost everywhere, and yet sufficiently scat- 

 tered among the creatures of the earth to excite wonder and admiration 

 at the novelty of the property. We can, of course, beg the question by 

 replying, " These creatures have the power of producing within them- 

 selves some chemical substance which, under certain circumstances, 

 produces light, probably as the result of oxidation," but this or equiva- 

 lent statements leave us very little nearer satisfaction than at first. 



The matter presents also another question which is difficult to 

 answer: Why should one creature be endowed with the photogenic 

 function, and yet some other form, closely related to the first, be un- 

 provided therewith? Did all creatures originally possess the power to 

 produce light and have all but the few we know lost this power, or have 

 the few that possess photogenicity acquired the power as a result of the 

 development of certain habits or conditions of life ? It seems probable 

 that both explanations may be advanced for different forms — i. e., that 

 in some cases related existing forms, some of which possess the photo- 

 genic function and others of which do not, are descended from a com- 

 mon photogenic ancestor, while in others the function has been devel- 

 oped during the history of the species. 



There are certainly three reasons for the existence of the photogenic 

 function some one of which is applicable to the great majority of 

 luminous creatures above the unicellular and very lowly organized 



