swimbladder. We can thus, as I infer from Professor Owen s interesting
description of these parts, understand the strange fact that every
particle of food and drink which we swallow has to pass over the
orifice of the trachea, with some risk of falling into the lungs,
notwithstanding the beautiful contrivance by which the glottis is
closed. In the higher Vertebrata the branchi have wholly
disappeared the slits on the sides of the neck and the loop-like course
of the arteries still marking in the embryo their former position. But
it is conceivable that the now utterly lost branchi 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 branchi 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 branchi , the whole surface of the body
and sack, including the small frena, serving for respiration. The
Balanid 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 branchi . Now I think
no one will dispute that the ovigerous frena in the one family are
strictly homologous with the branchi 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 branchi , 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 branchi in this latter family had
originally existed as organs for preventing the ova from being washed
out of the sack?
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