DIFFICULTIES OF THE THEORY 199 



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 wc 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 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 in- 

 heritance from a common ancestor; and in its absence in 

 some of the members to loss through disuse or natural selec- 

 tion. 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 geolog}' at all 

 lead to the belief that most fishes formerly possessed electric 

 organs, which their modified descendants have now lost. But 

 when we look at the subject more closely, we find in the sev- 

 eral fishes provided with electric organs, that those are situ- 

 ated in different parts of the body, — that they differ in con- 

 struction, 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 different 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 homol- 

 ogous, 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, arising 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. 



The luminous organs which occur in a few insects, belong- 

 ing to widely different families, and which are situated in 



