CHAP. VI. J OF NATURAL SELECTION. 141 



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 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 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 tho 

 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 several fishes provided 

 with electric organs, that these are situated in different parts ot 

 the body, that they differ in construction, as in the arrangement A 

 of the plates, and, according to Pacini, in the process or means by 

 which the electricity is excited and lastly, in being supplied witlr 

 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 

 homologous, 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, belonging to xi, 

 widely different families, and which are situated in different parts 

 of the body, offer, under our present state of ignorance, a difficulty 

 almost exactly parallel with that of the electric organs. Other 

 similar cases could be given; for instance in plants, the very 

 i curious contrivance of a mass of pollen-grains, borne on a foot- 

 stalk with an adhesive gland, is apparently the same in Orchis 

 and Asclepias, genera almost as remote as is possible amongst 

 Howering plants; but here again the parts are not homologous. 

 In all cases of beings, far removed from each other in the scale of 

 organisation, which are furnished with similar and peculiar organs, 



