LIVING FORMS AND LIGHT 537 



both fish and cuttlefish the explanation of these body-organs is prob- 

 ably the same. In the depths of the sea colors are practically indis- 

 tinguishable. The dim light of the stationary photogenic forms there 

 would be insufficient to differentiate colors. But in the more than semi- 

 darkness of these depths, a creature with a row of luminous dots along 

 its sides would show up like an electric sign in a dark street. Tbe ar- 

 rangement of the light-giving spots could very easily be followed, even 

 for considerable distances through the water. It is probable, therefore, 

 that these rows of photogenic organs on the bodies of these creatures 

 serve the purpose of plumage and pigmentation on land, a welcome for 

 friend and a warning for foe. No doubt one species of fish, seeing a 

 luminous streak some distance away through the water, could readily 

 tell whether its pattern of light — its " electric sign " — spelled the same 

 as those upon its own body, or the legend of a foe. Many of the fish 

 that possess these rows of luminous organs, while insignificant in size, 

 are obviously raptatorial, and are armed with vicious teeth. 



Of course another expression of the luminosity of deep-sea as well 

 as surface fish is that it is alluring. This is especially true in those 

 species which are provided with luminous barbels, or baits, like those of 

 the angler-fishes. Alcock mentions a blind angler-fish with a luminous 

 barbel, in which the alluring significance is scarcely to be doubted. 



An objection which seems to have been urged against the " pattern " 

 theory of photogenicity in marine forms is that different specimens of 

 the same species occasionally show variations in the number and dis- 

 tribution of their lights. It seems probable, however, that the reason 

 for this variation may be the age of the specimens of the fish in ques- 

 tion, the number or position of the spots varying with age. Almost any 

 of the various books and papers on the deep-sea fish will illustrate the 

 effect of this " pattern " arrangement as seen, for example, in Cyclo- 

 thone and Astronesthes. 



Still another type of organ of light-production among fishes is illus- 

 trated in Carl Chun's book " Aus den Tief en des Weltmeeres " ; in this 

 case the luminous apparatus is set in a pit in the foremost part of the 

 head, before the eyes. Here there can be little doubt, again, that the 

 usefulness to the animal consists in the illumination of the " road 

 ahead," as the searchlight does for the automobile. 



A large number of other forms might be mentioned, which emit 

 light in more or less characteristic ways, but what has gone before will 

 serve to illustrate the majority of the points in interest. Some consid- 

 erations as to the phenomenon in the fireflies may, however, be of in- 

 terest, especially as deductions therefrom will doubtless hold true for 

 many other luminous forms also. 



It has been observed that the various species of fireflies (Lampyrids) 

 emit lights of slightly differing tone, and in decidedly different man- 



voi,. lvxix.— 37. 



