72 
THE FERN FORESTS OF 
mospliere must have been already established, 
with all the attendant phenomena of light, heat, 
air, moisture, etc., yet it is probable that this 
atmosphere differed from ours in being very 
largely charged with carbonic acid. 
We should infer this from the nature of the 
animals characteristic of the period ; for, though 
land-animals were introduced, and the organic 
world was no longer exclusively marine, there 
were as yet none of the higher beings in whom 
respiration is an active process. In all warm- 
blooded animals the breathing is quick, requiring 
a large proportion of oxygen in the surrounding 
air, and indicating by its rapidity the animation 
of the whole system ; while the slow-breatliing, 
cold-blooded animals can live in an air that is 
heavily loaded with carbon. It is well known, 
however, that, though carbon is so deadly to 
higher animal life, plants require it in great 
quantities ; and it would seem that one of the 
chief offices of the early forests was to purify the at- 
mosphere of its undue proportion of carbonic acid, 
by absorbing the carbon into their own substance, 
and eventually depositing it as coal in the soil. 
Another very important agent in the process 
of purifying the atmosphere, and adapting it to 
the maintenance of a higher organic life, is found 
in the deposits of lime. My readers will excuse 
me, if I introduce here a very elementary chemi- 
