LIVING FORMS AND LIGHT 533 



forms. The first of these is that it is a secondary sexual character; 

 the second, it is protective in purpose; third, it is a lure for prey. 

 Other less general or less obvious uses may be shown, but in the great 

 majority of cases one or the other of these explanations of the use of 

 the light-producing function will be found adequate, and sometimes 

 more than one will apply to a given species. 



The simplest luminous forms are the bacteria. It is certainly diffi- 

 cult to see that photogenicity can be of any especial service to these 

 unicellular organisms. Most, if not all, of the photogenic bacteria are 

 cf marine origin, and it is possible that some of them may be pathogenic 

 to certain marine creatures when ingested, and the light thus serves to 

 warn the bacterivorous plankton, etc., that these bacteria are dangerous 

 food. Some species have been found to be pathogenic to Talitrus, a 

 crustacean, which may serve as a case in point. 



Luminous fungi are quite common, and here again we must leave 

 the question with at most only a poor attempt at explanation. Pro- 

 tective or warning it may be; but the author has never heard of lumi- 

 nosity in the most poisonous of all fungi, the Amanitas, and certainly 

 the luminous fungi he has seen did not appear to form any exception 

 to the usual fate of fungi in general of being abundantly attacked by 

 various species of insects. 



In both of these cases, the photogenicity may be like the fluorescence 

 of extracts of the common firefly, Photinus pyralis, and of cultures of 

 Bacillus fluorescens liquefaciens — merely a property of some chemical 

 substance elaborated by the life processes of the organism, and having 

 no bearing upon its economy. 



In simple marine forms like Noctiluca miliaris, the Pyrocystce, 

 Pyrosoma, etc., it is possible, though unlikely, that the luminosity has 

 a warning significance and it is obviously not a sexual character. These 

 creatures exist in swarms, or in the form of communities — compound, 

 or rather composite animals — and it would appear very probable that in 

 them the possession of the power to produce light finds its usefulness in 

 the fact that by its means they are enabled to communicate in such ways 

 as their low state of organization may require. 



It is perhaps a digression, but a few words may not be amiss as to 

 why the emission of light would be more useful to marine creatures 

 than some other modes of communication in use among land forms. 

 The sea- water is full of currents, ever changing and wavering ; it varies 

 in density slightly in different portions, owing to slight variation in 

 concentration and in temperature. Therefore, the emission of a sub- 

 stance to produce olfactory or gustatory sensations would be of little 

 use as a means of communication, especially to a creature capable of 

 motion, as many luminous forms are ; they would have far more control 

 over their own movements than over that of any emission. The pro- 



