No. 520] ELECTRICITY TISSUES IN FISHES 201 



packed out of the way at right angles to the axis of the 

 electroplax which remains the same as the former axis of 

 the muscle cells that were used to form it. 



The only difference lies in the fact that the striation 

 of the fibrils is retained in the Mormyrus forms while it is 

 lost in Gymnarchus, the dark-staining anisotropic sub- 

 stance apparently dissolving away. 



From what little can be predicted concerning the possi- 

 ble origin of the electric tissue in the other teleost forms 

 it is probable that the Mormyridae (including Gymnar- 

 chus) are the only fish in which the electroplax is formed 

 as a syncytium from more than one cell. In Astroscopes, 

 Electrophorus (forming Gymnotus) and Malopterurus 

 the structures show every evidence of having been devel- 

 oped from single myoblasts with the exception of Malop- 

 terurus, where it is a question as to whether they are not 

 evolved from gland cells instead. 



The evolution of these structures was most probably 

 not based upon a natural selective basis. It is true that 

 all muscle cells produce a slight static discharge at the 

 moment of contraction, and that the far greater shock 

 given by the electroplax is possibly a development of 

 this same discharge. But Darwin in his " Origin of 

 Species " has already said that the electric organs of 

 fishes were one of the serious obstacles in the way of his 

 natural selection theory, showing that the very slight dis- 

 charge of the more primitive organs could not possibly 

 have been useful to their possessors to the extent of an 

 excluding selection based on their presence or degree of 

 development. 



Some good evidence as to the methods of evolution 

 ought to be deduced from the degree of specialization and 

 distribution of electric organs in some of the groups; 

 even if experimental work seems at present to be impossi- 

 ble. In the skates, for instance, we find a very even and 

 general distribution of an organ and tissue that is ap- 

 parently in course of evolution, but which has not yet 

 arrived at a state of efficiency. It seems that the organ 

 must have originated in the common ancestor of the thirty 

 or more species of skates found in the seas of the world. 



