192 
DIFFICULTIES ON THEORY. 
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 conclud¬ 
ing that any organ could not possibly have been pro¬ 
duced 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 remarked. 
