In all this there is nothing abrupt; the changes shade into each other by insensible degrees.
How could it be otherwise?
The hot-blooded animals could not exist in an atmosphere so laden with carbonic acid as was that of the primitive times.

But the removal of that noxious ingredient from the air by the leaves of plants under the influence of sunlight, the enveloping of its carbon in the earth under the form of coal, the disengagement of its oxygen, permitted their life.

As the atmosphere was thus modified, the sea was involved in the change; it surrendered a large part of its carbonic acid, and the limestone hitherto held in solution by it was deposited in the solid form.

For every equivalent of carbon buried in the earth, there was an equivalent of carbonate of lime separated from the sea--not necessarily in an amorphous condition, most frequently under an organic form.

The sunshine kept up its work day by day, but there were demanded myriads of days for the work to be completed.