e work, being aided during
the progress of modification by the other organ; and then this other
organ might be modified for some other and quite distinct purpose, or be
wholly obliterated.

The illustration of the swim-bladder in fishes is a good one, because
it shows us clearly the highly important fact that an organ originally
constructed for one purpose, namely flotation, may be converted into
one for a widely different purpose, namely respiration. The swim-bladder
has, also, been worked in as an accessory to the auditory organs
of certain fishes. All physiologists admit that the swim-bladder is
homologous, or "ideally similar" in position and structure with the
lungs of the higher vertebrate animals: hence there is no reason to
doubt that the swim-bladder has actually been converted into lungs, or
an organ used exclusively for respiration.

According to this view it may be inferred that all vertebrate animals
with true lungs are descended by ordinary generation from an ancient
and unknown prototype which was furnished with a floating apparatus or
swim-bladder. 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 branchiae have wholly
disappeared--but in the embryo the slits on the sides of the neck and
the loop-like course of the arteries still mark their former position.
But it is conceivable that the now utterly lost branchiae might have
been gradually worked in by natural selection for some distinct purpose:
for instance, Landois has shown that the wings of insects are developed
from the trachea; it is therefore highly probable that in this great
class organs which once served for respiration have been actually
converted into organs for 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 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 sm