OF NATURAL SELECTION. 169 



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 

 aie 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 proces: 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 func- 

 tion. 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, disap- 

 pears, 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 

 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 among flowering 

 olants ; 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 although the general appearance 

 and function of the organs may be the same, yet fundamental 

 differences between them can always be detected. For in- 

 stance, the eyes of Cephalopods or cuttle-fish and of verte- 

 brate 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 throwing an image at the back 

 of a darkened chamber. Beyond this superficial resemblanee, 

 there is hardly any real similarity between the eyes of cuttle 



