DIFFICULTIES ON THEORY 173 



family ; indeed, they graduate into each other. There- 

 fore I do not doubt that little folds of skin, which 

 originally served as ovigerous frena, but which, like- 

 wise, very slightly aided the act of respiration, have 

 been gradually converted by natural selection into 

 branchiae, simply through an increase in their size and 

 the obliteration of their adhesive glands. If all ped- 

 unculated cirripedes had become extinct, and they 

 have already suffered far more extinction than have 

 sessile cirripedes, who would ever have imagined that 

 the branchiae in this latter family had originally existed 

 as organs for preventing the ova from being washed 

 out of the sack ? 



Although we must be extremely cautious in con- 

 cluding that any organ could not possibly have been 

 produced by successive transitional gradations, yet, un- 

 doubtedly, grave cases of difficulty occur, some of which 

 will be discussed in my future work. 



One of the gravest is that of neuter insects, which 

 are often very differently constructed from either the 

 males or fertile females ; but this case will be treated 

 of in the next chapter. The electric organs of fishes 

 offer another case of special difficulty ; it is impossible 

 to conceive by what steps these wondrous organs have 

 been produced ; but, as Owen and others have re- 

 marked, their intimate structure closely resembles that 

 of common muscle ; and as it has lately been shown 

 that Rays have an organ closely analogous to the 

 electric apparatus, and yet do not, as Matteucei asserts, 

 discharge any electricity, we must own that we are far 

 too ignorant to argue that no transition of any kind is 

 possible. 



The electric organs offer another and even more 

 serious difficulty ; for they occur in only about a dozen 

 fishes, of which several are widely remote in their 

 affinities. Generally when the same organ appears in 

 several members of the same class, especially if in 

 members having very different habits of life, we may 

 attribute its presence to inheritance from a common 

 ancestor ; and its absence in some of the members to 



