1887.] on some Electrical Fishes. 141 



by some external agency capable of exciting it. Another established 

 fact is that the effect is of limited duration, and that for its develop- 

 ment a certain time must elapse, which under similar conditions is 

 always the same for the same structure. A third is that all kinds of 

 excitants act in the same way, the effects differing in intensity, not 

 in direction. In all these respects, and in others of less importance, 

 the electrical plate agrees with muscle and nerve. Inasmuch, there- 

 fore, as we have met wdth a structure, of which the development of 

 electrical action is the exclusive function, there seems to be good 

 reason for the hope that by its investigation, a nearer approach may 

 be made than has hitherto been possible to the central question — 

 that of the reason why in all animal structures the transition from 

 the inactive to the active state is, so far as our present knowledge 

 teaches, always accompanied by electrical change. 



The question why certain fish are endowed wdth so singular a 

 means of offence and defence, which others allied to them zoologi- 

 cally do not j)ossess, and above all, w^hy some fish have electrical 

 organs so small as to be useless, is as difficult to answer now as 

 when Mr. Darwin wrote the ' Origin of Species.' The facts relating 

 to the development of the organ, which teach us to regard it as, in 

 some sense, a modified muscle, might suggest that the transition from 

 muscle to organ was a gradual one determined by external conditions. 

 But -we are prevented from accejDting any such suggestion by the 

 consideration that an electrical organ only becomes advantageous to 

 its possessor when it has acquired sufficient size to be used in the 

 capture of prey, and that in all previous stages of transition it must 

 be useless. Natural selection could not therefore determine the 

 development of the electrical organ by modification of muscle. It 

 is more reasonable to imagine that all fish, or at any rate certain 

 families of fish, possess an undevelo23ed element of structure, of 

 which the electrical organ is the manifestation. So that what we 

 have to account for is not its presence in some exceptional cases, but 

 its absence in the great majority. 



The existence of such a tendency as this hypothesis supposes, 

 would render it possible for natural selection to operate efficiently in 

 bringing about the observed result. 



[J. B. S.] 



