910 LIGHT AND LIFE 



as a physical requirement for the emission ot light. There are many 

 photogenic structures in the animal kingdom that are not in the least 

 eyelike; and luminous bacteria, of course, emit light with no need 

 for accessory structures at all. 



In the course of evolution bioluminescence has appeared inde- 

 pendently in many groups, and in contrast to eyes, which are almost 

 ubiquitous among the higher animals, photophores appear sporadi- 

 cally. Harvey concluded that bioluminescence generally arose as an 

 accidental aberration in some conventional biochemical system. The 

 value of producing light to species which do so is not even clear in 

 any case except the fireflies, although for deep-sea forms it is proba- 

 ble that light serves as a recognition signal for schooling and mating. 

 At least it seems to be connected with an ocular response on the part 

 of other individuals of the same species. If so, vision would necessarily 

 antedate bioluminescence and would provide the basis for the selec- 

 tion pressure that would establish and elaborate the photogenic ca- 

 pacity, once it appeared. A presumptive association of eyes and photo- 

 phores is strengthened by the fact that there are many cases among 

 shrimps and molluscs of eyes without photophores, but none of 

 photophores without eyes. And in sergestid shrimps, the four species 

 with photophores have the largest eyes. Then have the eyelike photo- 

 phores evolved from eyes? Could a structure perhaps at first function 

 simultaneously as eye and photoj^hore? Perhaps a comparative study 

 of these photogenic organs might afford a better understanding of the 

 evolutionary origin of novelties. A species with supernumerary eyes, 

 or the capacity to jiroduce such by a homoeotic transformation of 

 serially homologous structures, might well find it possible to con- 

 vert supernumerary eyes to photogenic organs if there was any selec- 

 tive advantage in possessing such organs. 1 he modification of eyes, 

 vertebrate or invertebrate, into photic organs is less strange, and 

 probably less difficult to achieve, than many other kinds of evolu- 

 tionary novelty, the first origin of conqjlex eyes, for exanqjlc. 



Conclusion 



The current trends in the scientific investigation of the relation ol 

 light to life will probably be quite clear to any reader of the present 

 Symposium volume. In spite of many interesting side-issues, the pre- 

 dominating theme appears to be the eflort to harness the primary 

 photochemical events that follow the absorption of a quantum of 

 light — the production of excited states in molecules — to the chemical 

 reactions known to ensue. The analysis of fluorescence and al^sorp- 



