192 DIFFICULTIES ON THEOKY. Chap. VI. 



called by me the ovigerous frena, which serve, through 

 the means of a sticky secretion, to retain the eggs until 

 they are hatched within the sack. These cirripedes 

 have no branchiae, the whole surface of the body and 

 sack, including the small frena, serving for respiration. 

 The Balanidae or sessile cirripedes, on the other hand, 

 have no ovigerous frena, the eggs lying loose at the 

 bottom of the sack, in the well-enclosed shell ; but they 

 have large folded branchiae. Now I think no one will 

 dispute that the ovigerous frena in the one family are 

 strictly homologous with the branchiae of the other 

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

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

 ginally served as ovigerous frena, but which, likewise, 

 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 oblite- 

 ration of their adhesive glands. If all pedunculated 

 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 pre- 

 venting 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 winch 

 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 remarked, 



