age 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 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 position. But
it is conceivable that the now utterly lost