Chap. VI.] OF NATUKAL SELECTION. 235 



muscle and nerve during rest, and the discharge of the 

 torpedo, instead of being peculiar, may be only another 

 form of the discharge which attends upon the action of 

 muscle and motor nerve." Beyond this we cannot at 

 present go in the way of explanation ; but as we know 

 so little about the uses of these organs, and as we know 

 nothing about the habits and structure of the progenitors 

 of the existing electric fishes, it would be extremely 

 bold to maintain that no serviceable transitions are 

 possible by which these organs might have been gradu- 

 ally developed. 



These organs appear at first to offer another and far 

 more serious difficulty ; for they occur in about a dozen 

 kinds of fish, of which several are widely remote in their 

 affinities. When the same organ is found in several 

 members of the same class, especially if in members 

 having very different habits of life, we may generally 

 attribute its presence to inheritance from a common 

 ancestor ; and its absence in some of the members to 

 loss through disuse or natural selection. So that, if the 

 electric organs had been inherited from some one 

 ancient progenitor, we might have expected that all 

 electric fishes would have been specially related to each 

 other ; but this is far from the case. Nor does geology 

 at all lead to the belief that most fishes formerly 

 possessed electric organs, which their modified descend- 

 ants have now lost. But when we look at the subject 

 more closely, we find in the several fishes provided with 

 electric organs, that these are situated in different parts 

 of the body, — that they differ in construction, as in the 

 arrangement of the plates, and, according to Pacini, in 

 the process or means by which the electricity is excited 

 —and lastly, in being supplied with nerves proceeding 



