CHAPTER VI. DIFFICULTIES OF THE THEORY.
5. MODES Of TRANSITION. (continued)
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 another 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 of the sack, together with 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, within the
well-enclosed shell; but they have, in the same relative position with the
frena, large, much-folded membranes, which freely communicate with the
circulatory lacunae of the sack and body, and which have been considered by
all naturalists to act as 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 it need not be doubted that the two little folds of skin, which
originally served as ovigerous frena, but which, likewise, very slightly
aided in 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 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?