2()2 THE ORIGIN OF SPECIES 



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 construc- 

 tion, 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 pro- 

 ceeding from different sources, and this is perhaps the 

 most important of all the differences. Hence in the sev- 

 eral 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 resem- 

 bled each other in all respects. Thus the difl&culty of an 

 organ, apparently the same, arising in several remotely 

 allied species, disappears, leaving only the lesser yet still 

 great difl&culty; 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 diflficulty almost exactly 

 parallel with that of the electric organs. Other similar 

 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 



