Chap. vi. of Natural Selection. *5 1 



its absence in some of the members to loss through^ disuse or 



natural selection. So that, if the electric organs nad been 

 Merited 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, *or 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 m dif- 

 ferent 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 homologous, but only as analogous in function. Con- 

 sequently 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 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 

 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 flowering 

 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, it will be 

 found that although the general appearance and function of the 

 organs may be the same, yet fundamental differences between them 

 can always be detected. For instance, the eyes of cephalopods 

 or cuttle-fish and of vertebrate animals appear wonderfully alike ; 

 and in such widely sundered groups no part of this resemblance can 

 be due to inheritance from a common progenitor. Mr. Mivart has 

 advanced this case as one of special difficulty, but I am unable to 

 see the force of his argument. An organ for vision must be formed 

 of transparent tissue, and must include some sort of lens for 



