Chap. VI. 
TRANSITIONS OF ORGANS. 
191 
which view is now generally held, a part of the audi¬ 
tory apparatus has been worked in as a complement to 
the swimbladder. 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 seems to me to be no great diffi¬ 
culty 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 ani¬ 
mals having true lungs have descended by ordinary 
generation from an ancient prototype, of which we know 
nothing, 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 dis¬ 
appeared—the slits on the sides of the neck and the 
loop-like course of the arteries still marking in the em¬ 
bryo their former position. But it is conceivable that 
the now utterly lost branchiae might have been gra¬ 
dually worked in by natural selection for some quite 
distinct purpose: in the same manner as, on the view 
entertained by some naturalists that the branchiae 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, 
