o, ,2 



THE AMERICAN NATURALIST 



[Vol. XLIV 



All the Bajidrc have this organ and yet their close allies 

 show no sign of its appearance. One form in particular 

 was formerly classed with the skates, but some years ago 

 was removed on anatomical evidences from this group by 

 D. S. Jordon and classed in another genus. The writer 

 dissected a specimen of this species with great interest 

 and care and found that there was no trace of electric 

 tissue. And yet this species is undoubtedly closely re- 

 lated to the skates and must have inherited at least their 

 potential powers to develop electric tissue. Furthermore 

 this species is skate-like to an extreme degree in form 

 and habit and must have lived under conditions and sur- 

 roundings similar to those which we must assume were the 

 ones, if any, that stimulated the skate ancestors to change 

 muscle tissue into electric tissue. Evidently there are 

 internal as well as external conditions and stimulse to be 

 taken into account. 



Likewise among the torpedoes ; all possess a very highly 

 developed organ, evidently a common inheritance from 

 some ancestor in which it originated. Subsequent varia- 

 tion in the tissue has not kept pace with the fair amount 

 of external variation in the several species. It seems 

 that the impulse to evolve this tissue lias extended into all 

 the members of these two groups, a real inner stimulus 

 working independently of outer condition. 



On the other hand, other groups of similar form, appear- 

 ance and mode of life to some of the other electric fishes 

 absolutely lack the electric tissues. These may or may 

 not be nearly related. A rather remarkable example is 

 to be seen in Astroscopes and its related form TJrana- 

 scopus scaber. These two fish look so much alike that 

 one may be used to show the fishermen what the other 

 looks like. Their habits are practically the same. Also 

 this feature is noticeable among the Gymnotidae, whose 

 various members show many grades of specialization but 

 have not been sufficiently studied as to the possession of 

 rudimentary electric organs. These studies together with 

 the histogenesis of other teleost electric tissues, particu- 

 larly that of Malopterurus, are most attractive fields for 

 future work. 



