764 LIGHT AND LIFE 



all those producing an extracellular luminous secretion, and all those 

 that produce light continuously or are known to be hormonally- 

 controlled — most of which in any case show no resemblance to eyes 

 whatsoever — there still remain many organs, even in deepwater 

 Crustacea and squids, that seem to function without most of the 

 structure that one might, a priori, have anticipated in a light-emitting 

 organ — for example, a translucent window on the outside. Only in 

 organs actually in the integument, where the need would seem to be 

 least, do "cornea" and lens become common. 



Evolutionary Implications 



Even if the anatomies of sighting and lighting have no relation to 

 one another, it may be stimulating to speculate about possible func- 

 tional interrelations during evolution. Concerning this we have no 

 shred of direct evidence, the fossil record having nothing to contribute 

 in relation to the temporal sequence of vision and photogeny nor to 

 the former existence of more primitive types or intermediate stages 

 of photophore structure. However, a good deal is known or reasonably 

 surmised about the evolution of eyes, and there is a wealth of indirect 

 evidence about photophore evolution. 



The distribution of bioluminescence in the animal kingdom leads, 

 by conventional paths of evolutionary thought, to the conclusion that 

 light-production must have appeared independently in a great num- 

 ber of different groups. Harvey (13) has documented this interesting 

 deduction in great detail, emphasizing the sporadic occurrence of the 

 faculty even among close relatives, and likening its almost random 

 distribution to what w^ould be obtained by throwing a handful of sand 

 over the phylogenetic tree, with each grain marking a luminous form. 

 The status of photophores thus contrasts strongly with that of eyes, 

 which, though similarly polyphyletic in origin, are almost ubiquitous. 

 From the above deduction Harvey has also drawn the corollary that 

 the ability to produce light must have arisen as accidental aberrations 

 in otherwise conventional biochemical systems. 



We can hardly come to grips with the evolution of photogeny with- 

 out considering the question of the function of the light — that is, 

 its selective value or use to the organism. Anthropopsychic judgments 

 of this sort are clearly fraught with pitfalls, but at least no more so 

 than in the case of vision, in relation to which, for example, the con- 

 ventional attitude would be that enviromnental light has acted over 

 the ages as a selective agent in evoking the camera eye independently 

 in annelids, molluscs, and vertebrates. Therefore, without any over- 



