Page:Popular Science Monthly Volume 79.djvu/536

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TO every observer of natural phenomena, the query must some time come, "Why do certain living creatures produce light?" The luminous 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 endowed with the property of producing light? The question is undoubtedly 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 scattered 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 themselves some chemical substance which, under certain circumstances, produces light, probably as the result of oxidation," but this or equivalent 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 unprovided 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 photogenic function and others of which do not, are descended from a common photogenic ancestor, while in others the function has been developed 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