PRODUCTION OF ELECTRICITY 437 



In the case of the torpedo the shock sustained by a single 

 person touching a healthy specimen at two points is strong 

 enough to be very painful, and if a number of persons join 

 hands and the person at each end touches the fish, a consider- 

 able shock passes through the whole series of persons. The 

 electric condition to which the shock is due is apparently pro- 

 duced by nervous stimulation at the moment of the discharge, 

 in consequence of the stimulation of the sensory nerves of the 

 fish : anything touching the skin of the fish causes by reflex 

 action stimulation of the electric nerves and so produces a 

 shock, just as in ordinary cases a sensation of touch, that is 

 stimulation of the sensory nerves of the skin, gives rise to a 

 muscular contraction ; and just as muscles and nerves become 

 exhausted by muscular contractions, so the electric organs and 

 their nerves become exhausted by discharges and only recover 

 their power after a period of rest. 



With regard to the evolution of electric organs, Darwin him- 

 self was much impressed by the difficulty of explaining their 

 origin on his theory of natural selection. The difficulty is not 

 so much the transitional stages from muscle to electric organs, 

 since in the skates we have in existence all stages of the transi- 

 tion, but to understand how the rudimentary stages could be or 

 have been of any use to the possessor, and therefore of what is 

 called selection value. We can understand that a well-developed 

 electric organ is of great use to its possessor either to paralyse 

 its enemies or its prey, but we have no evidence that the slight 

 powers of the organs in skates are of any use to those fish, since 

 for some time electricians were unable to obtain any effects from 

 them on electric apparatus. Even in the Mormyridae the earlier 

 experimenters were unable to obtain any electric effects from 

 the organs. The theory of natural selection then affords no ex- 

 planation of the evolution or these organs, as the accumulation 

 of slight variations in the earlier stages seems impossible. On 

 the other hand, we cannot at present point to any conditions of 

 life which would produce directly the transformation of muscular 

 tissue into electrical organs. We can only ponder over the 

 apparent parodox that the increased stimulation and develop- 

 ment of nerves that were originally motor should have been 

 accompanied by the degeneration of the contractile property of 

 the muscular tissue, considering that stimulation of such nerves 



