96 THE STRUCTURE AND LIFE OF BIRDS cuap, 
speaking, equal to the amount of oxygen absorbed.. 
M. Milne Edwards records some experiments of this 
nature, the results of which are very: striking! The 
amount of carbonic acid gas exhaled by various 
animals during a given time was exactly measured, 
and then equated to one standard, so that the different 
sizes of the subjects of the experiments might cause 
no confusion. Thus, the figures that follow enable us 
to compare animals of the most widely separated 
classes, in respect of the amount of carbonic acid which 
they breathe out ; and since, as I have said, this roughly 
corresponds to the amount of oxygen absorbed, it is a 
measure of the excellence, or the reverse, of their 
breathing apparatus. 
The slug ... ae aed: 112 
The snail ... diss we 4 0r 5 
The toad ... ae w. 5 0r 6 
The frog ... Dees . 7or 8 
The guinea-pig ... .. 14 Or 15 
The pigeon igs si .. 20 
The pigeon is a good deal ahead of the guinea-pig, 
the only other warm-blooded animal in the list. The 
cold-blooded creatures are far behind the guinea-pig. 
To sum up, then, a bird’s respiratory system is, as 
far as we know, much more active than that of any 
mammal. As evidence of this we have—(1) the greater 
amount of carbonic acid given off ; (2) the more rapid 
breathing, the effect of which is much increased by the 
air-sacks ; (3) high temperature, which could not exist 
without thorough oxidation of the blood. 
1 Physiologie et Anatomie comparée, vol. ii., p. 534.- 
