all 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?

There is another possible mode of transition, namely, through the
acceleration or retardation of the period of reproduction. This has
lately been insisted on by Professor Cope and others in the United
States. It is now known that some animals are capable of reproduction
at a very early age, before they have acquired their perfect characters;
and if this power became thoroughly well developed in a species, it
seems probable that the adult stage of development would sooner or later
be lost; and in this case, especially if the larva differed much from
the mature form, the character of the species would be greatly changed
and degraded. Again, not a few animals, after arriving at maturity, go
on changing in character during nearly their whole lives. With mammals,
for instance, the form of the skull is often much altered with age, of
which Dr. Murie has given some striking instances with seals. Every
one knows how the horns of stags become more and more branched, and the
plumes of some birds become more finely developed, as they grow older.
Professor Cope states that the teeth of certain lizards change much in
shape with advancing years. With crustaceans not only many trivial,
but some important parts assume a new c