In other
words, the primitive atmosphere would allow the heat of the sun
to penetrate it, and then, as the ground absorbed the light,
would retain a large proportion of the heat. Hence the
semi-tropical nature of the primitive earth, the moisture, the
dense clouds and constant rains that are usually ascribed to it.
This condition lasted until the rocks and the forests of the
Carboniferous age absorbed enormous quantities of carbon-dioxide,
cleared the atmosphere, and prepared an age of chill and dryness
such as we find in the Permian.
But the planetesimal hypothesis has no room for this enormous
percentage of carbon-dioxide in the primitive atmosphere. Hinc
illoe lachrymoe: in plain English, hence the acute quarrel about
primitive climate, and the close scanning of the geological
chronicle for indications that the earth was not moist and warm
until the end of the Carboniferous period. Once more I do not
wish to enfeeble the general soundness of this account of the
evolution of life by relying on any controverted theory, and we
shall find it possible to avoid taking sides.
I have not referred to the climate of the earth in earlier ages,
except to mention that there are traces of a local "ice-age"
about the middle of the Archaean and the beginning of the
Cambrian.
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