branchiae might have
been gradually worked in by natural selection for some quite distinct
purpose: in the same manner as, on the view entertained by some
naturalists that the branchiae and dorsal scales of Annelids are
homologous with the wings and wing-covers of insects, it is probable
that organs which at a very ancient period served for respiration have
been actually converted into organs of flight.

In considering transitions of organs, it is so important to bear in mind
the probability of conversion from one function to another, that I will
give one more instance. Pedunculated cirripedes have two minute folds of
skin, 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. Therefore I do not doubt that little folds of
skin, which originally 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 obliteration 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 preventing the ova from being washed
out of the sack?

Although we must be extremely cautious in concluding that any organ
could not possibly have been produced by successive transitional
gradations, yet, undoubtedly, 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 orga