osition.
But it is conceivable that the now utterly lost 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, {192} 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; i