200 THE HAUNTS OF LIFE 
NEW WAYS OF BREATHING 
The colonisation of the dry land by aquatic 
animals cannot have been an easy task, and 
our question now is: What were the neces- 
sary qualifications? 
The first qualification was ability to capture 
the oxygen of the dry air. There is a much 
larger proportion of oxygen in the air than 
there is mixed with the water, but it is not so 
readily available. For, mixed with the water, 
it seems to seep in very readily through the 
delicate moist skin of the general surface of 
the body, or of special organs, suchas gills. On 
one side of the membrane there is water, with 
oxygen mixed in it; on the other side of the 
membrane there is blood, which usually car- 
ries a pigment with a strong afhnity for oxy- 
gen. Whathappensin aquatic breathing is that 
the oxygen diffuses through the skin into the 
blood, usually entering into a loose, chemical 
union with the blood-pigment. With its cap- 
tured oxygen the blood passes to the living tis- 
sues of the animal, to the muscles, for instance, 
and there surrenders its oxygen to keep up the 
ceaseless burning (or oxidation) which living 
implies. As the result of the combustion (or 
