72 INTRODUCTION. 



has even presumed, that a certain red, fleshy- 

 looking substance, which is often found within 

 it, acts in the manner of a gland, and secretes 

 from the blood the air which it contains. It 

 seems fair to conclude, then, that at least a 

 great number of fishes rise in the water by means 

 of their air-bladder, not by removing from this 

 organ a part of its accustomed pressure, and thus 

 rarefying the air which it contains, but by deposit- 

 ing more air within it ; and that they sink in the 

 water, not by condensing this air, as the result of 

 the Increased pressure to which they subject the 

 air-bladder, but by getting rid of a portion of it : 

 and if this be certainly the ease in so many fishes, 

 analogy would render it probable that it is so in 

 all; and that the only difference between those 

 which have, and those which have not a passage 

 from their air-bladder, is, that in the former the 

 pressure exercised directly expels the air, while, in 

 the latter, it promotes its absorption. It has been 

 contended, that the floating of fishes after death is 

 a proof that they rise in the water, during life, 

 merely by relieving the air-bladder from its ordinary 

 pressure, and not by any active process ; but this 

 argument is very fallacious, since fishes in general, 

 unlike the cetaceous animals, Avhich are naturally 

 lighter than water, do not float till some time 

 after death; and, when they do so, it is as the 

 result of a quantity of new gasses formed in their 

 body by putrefaction, precisely as occurs with man 

 and terrestrial animals in general. Besides, if it 



