191 



THE AMERICAN NATURALIST 



[Vol. XLIV 



Of the other two abilities, that to produce light and to 

 generate electricity, we find that both are rare. Lumi- 

 nosity is found to be widely distributed among the various 

 kinds of animals, most of the principal groups having one 

 or more representatives which can produce light. And 

 yet this is, after all, a rare specialization and the percent- 

 age of animals which are luminous is very small indeed. 

 It is not so absolutely necessary as the power to move 

 nor can it be regarded as having the selective value that 

 the possession of heat-producing tissues must have. Its 

 real function, sexual, warning, or for purposes of seeing 

 objects in the dark, has not been satisfactorily determined 

 for any groups except possibly the deep-sea fishes and 

 even here we find more to be without it than with it. 



As to the last of the dynamic tissues, that which pro- 

 duces electricity, we have here a tissue which is remark- 

 able to an extreme degree both for the rarity of its occur- 

 rence and its narrow distribution, is being confined to one 

 group of vertebrates, the fishes, and found here among 

 only seven families. 



In these seven groups the tissue seems to have devel- 

 oped absolutely independently and to have formed seven 

 separate types of organ differing markedly from one an- 

 other in details of structure and position of the organ and 

 yet all adapted, through certain analogous developments, 

 to perform the same function. 



We evidently have here a tissue of comparatively 

 recent origin, phylogenetically, and one of the first studies 

 to be made toward a knowledge of its evolution is to find 

 out what can be learned from its history in the individual 

 or its histogenesis. 



Of the seven types of organs two are found in elasmo- 

 branchs and the remaining five in teleost fishes. The 

 histogenesis of the two elasmobranch forms has been 

 worked out by Ewart, Engelmann, Babuchin, Ogneff and 

 others, but the corresponding history of this tissue in the 

 teleosts has remained unproved with the exception of one 

 form, Gymnarchxis, the ontogenetic origin of whose elec- 

 tric organ the writer presents below. 



