OF NATURAL SELECTION. I79 



nerve during the 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 gj 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 gradually developed. 



These organs appear at first to ofEer 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 mem- 

 bers 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 for- 

 merly possessed electric organs, which their modified 

 descendants have now lost. But when we look at the sub- 

 ject 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 from dif- 

 ferent sources, and this is perhaps the most important of 

 all the differences. Hence in the several fishes furnished 

 with electric organs, these cannot be considered as hom- 

 ologous, but only as analogous in function. Consequently 

 there is no reason to suppose that they have been inherited 

 from a common progenitor; for had this been the case they 

 would have closely resembled each other in all respects. 

 Thus the difficulty of an organ, apparently the same, aris- 

 ing in several remotely allied species, disappears, leaving 

 only the lesser yet still great difficulty: namely, by what 

 graduated steps these organs have been developed in each 

 separate group of fishes. 



