Though the existence of such an extensive
terrestrial vegetation shows conclusively that an atmosphere 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-breathing,
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 atmosphere 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.
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