two of the means, or even all three, combined in the same individual. In
all such cases one of the two organs might readily be modified and
perfected so as to perform all the 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