154 THE ORIGIN OF SPECIES 



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, espe- 

 cially if in members having very different habits of life, we may 

 generally attribute its presence to inheritance from a common an- 

 cestor; 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 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 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 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 ho- 

 mologous, but only as analogous in function. Consequently there 

 is no reason to suppose that they have been inherited from a com- 

 mon 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 

 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 simi- 

 lar cases could be given; for instance in plants, the very 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 among flowering plants ; but 

 here again the parts are not homologous. In all cases of beings, 

 far removed from each other in the scale of organization, which 

 are furnished with similar and peculiar organs, it will be found that 



