420 THE CANADIAN NATURALIST. [Dec. 



more rapidly does waste occur, and the greater necessity is there 

 for rapid renewal. One of the results of waste in the animal 

 economy is, the liberation of carbonic acid ; which carbonic acid 

 is produced by the union of the broken down carbonaceous par- 

 ticles of the old body with a portion of oxygen still existing in 

 the blood. Unless this poisonous carbonic (when thus formed) 

 be speedily removed, death is the inevitable consequence. 



Thus arises the paramount necessity for the exercise of this 

 function of respiration — which consists essentially in the removal 

 of carbonic acid from the fluids of an animal's body, and in the 

 interchange for this of an equivalent amount of oxygen. The 

 mode by which this is effected, is wonderfully varied in different 

 classes of animals ; the respiratory apparatus of each great division 

 being beautifully adapted to the peculiar mode of such animal's 

 existence, and to the general plan of its structure. But in all 

 cases, however complicated may be the structural arrangements 

 this function is performed, it depends essentially on the effective 

 action of a most exquisitely simple law, usually expressed as that 

 of ' the diffusion of gases.' Thus : — if a bladder containing 

 pure oxygen gas be hung up in a room or vessel containing com- 

 mon atmospheric air, although no distinguishable pores may exist 

 in the membraneous bag thus containing the gas, still, after a 

 while, an interchange will have taken place between the internal 

 and external gases ; and the bladder will ultimately be found to 

 contain nothing but common air ! This interchange will take 

 place between other dissimilar gases under the same conditions ; 

 and thus, the beautifully simple arrangement is provided for the 

 carrying on of this all important function of respiration. For 

 it matters not whether an animal may exist in the water or on 

 the land ; whenever or however the blood (which may have be- 

 come overcharged with carbonic acid by its passage through the 

 body) is brought, through the intervention of an enclosing mem- 

 brane, in contact with oxygen, contained either in water or in the 

 air, this interchange — of which we are speaking— instantly takes 

 place, and respiration, or -the revivifying of the blood, is the re- 

 sult. It would have been interesting to trace the various struc- 

 tural arrangements by which this is effected in different grades of 

 animal life ; but this would lead us too far away from the special 

 subject under consideration. It will, however, be necessary to 

 make a passing reference to the respiratory apparatus of other 

 animals ; in order to show clearly the totally distinct, and very 



