AMERICAN NATURALIST 
Vou XXVI. December, 1892. eT 312 
THE ORIGIN OF LUNGS, A CHAPTER IN EVOLUTION. 
By CHARLES MORRIS. 
The air bladder of fishes is an organ whose true purpose has 
long been classed among the mysteries of animal organization. 
All we know about it is the duty to which it is now occasion- 
ally devoted ; but there is abundant reason to believe that this 
was not its original function. This duty is indicated by its 
frequent title of “swim-bladder,” the organ being seemingly 
used to some extent to aid the fish in swimming. Cuvier tells 
us that “the most obvious use of the swim-bladder is to keep 
the animal in equilibrium with the water, or to increase or 
reduce its relative weight, and thereby cause it to ascend or 
sink, in proportion as that organ is dilated or compressed.” 
In addition it may be of use, as Günther observes, “in raising 
the fore-part of the body or depressing it, as occasion may 
require.” 
Doubtless all this is correct, but that the bladder was 
_ evolved for such a purpose, or is of any essential utility as a 
swimming organ, there is the strongest reason to question ; 
and in all probability its original purpose was something 
quite different. As it at present exists it is often too small to 
be of use in changing the gravity of the fish, and in many 
cases it is entirely absent. In others, its compressing muscles, 
as Van der Hæven states, seem incapable of being used to 
expand it. Yet in all these cases the fish seems at no disadvan- 
tage in swimming as compared with those that possess large and 
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