tural selection might easily specialise, if any advantage were thus
gained, a part or organ, which had performed two functions, for one
function alone, and thus wholly change its nature by insensible steps. Two
distinct organs sometimes perform simultaneously the same function in the
same individual; to give one instance, there are fish with gills or
branchiae that breathe the air dissolved in the water, at the same time that
they breathe free air in their swimbladders, this latter organ having a
ductus pneumaticus for its supply, and being divided by highly vascular
partitions. In these cases one of the two organs might with ease be
modified and perfected so as to perform all the work by itself, being aided
during the process of modification by the other organ; and then this other
organ might be modified for some other and quite distinct purpose, or be
quite obliterated.

The illustration of the swimbladder 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 wholly different purpose, namely respiration. The swimbladder has,
also, been worked in as an accessory to the auditory organs of certain
fish, or, for I do not know {191} which view is now generally held, a part
of the auditory apparatus has been worked in as a complement to the
swimbladder. All physiologists admit that the swimbladder is homologous, or
"ideally similar" in position and structure with the lungs of the higher
vertebrate animals: hence there seems to me to be no great difficulty in
believing that natural selection has actually converted a swimbladder into
a lung, or organ used exclusively for respiration.

I can, indeed, hardly doubt that all vertebrate animals having true lungs
have descended by ordinary generation from an ancient prototype, of which
we know nothing, furnished with a floating apparatus or 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 branchiae 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 p